View source for War

{{Short description|Intense armed conflict}}
{{redirect-several|War|Wars|Warring|Warfare|Conflict zone}}

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{{multiple image|perrow = 2|total_width=300
| image1 = Stele of Vultures detail 01a.jpg
| alt1 = Part of the Stele of the Vultures depicting heavy infantry marching in formation
| image2 = Bayeuxtapestryscene52.jpg
| alt2 = Part of the Bayeux Tapestry depicting Norman heavy cavalry charging Saxon shield wall
| image3 = Nagasakibomb.jpg
| alt3 = Nuclear mushroom cloud
| image4 = Napoleons retreat from Moscow by Adolph Northen.jpg
| alt4 = Painting of Napoleon and his troops in winter retreating from Moscow
| image5 = Into the Jaws of Death 23-0455M edit.jpg
| alt5 = Soldiers wading ashore from landing craft on D-Day
| image6 = British Mark I male tank Somme 25 September 1916.jpg
| alt6 = British rhomboid tank and soldiers preparing to advance
| footer = Clockwise from top-left:<br />Ancient warfare: [[Stele of the Vultures]], {{circa}} 2500 BCE<br />Medieval warfare: [[Battle of Hastings]], 1066<br />Early modern warfare: [[Retreat from Moscow]], 1812<br />Industrial age warfare: [[Battle of the Somme]], 1916<br />Modern warfare: [[Normandy landings]], 1944<br />Nuclear warfare: [[Atomic bombing of Nagasaki]], 1945}}
{{War}}
'''War''' is an armed conflict{{refn|group=lower-alpha|The term "armed conflict" is used instead of, or in addition to, the term "war" with the former being more general in scope. The [[International Committee of the Red Cross]] differentiates between international and non-international armed conflict in their definition, "International armed conflicts exist whenever there is resort to armed force between two or more States.... Non-international armed conflicts are protracted armed confrontations occurring between governmental armed forces and the forces of one or more armed groups, or between such groups arising on the territory of a State [party to the Geneva Conventions]. The armed confrontation must reach a minimum level of intensity and the parties involved in the conflict must show a minimum of organisation."<ref>{{cite web|title=How is the Term "Armed Conflict" Defined in International Humanitarian Law?|url=https://www.icrc.org/eng/assets/files/other/opinion-paper-armed-conflict.pdf|website=International Committee of the Red Cross|date=March 2008|access-date=7 December 2020|archive-date=1 November 2018|archive-url=https://web.archive.org/web/20181101193920/https://www.icrc.org/eng/assets/files/other/opinion-paper-armed-conflict.pdf|url-status=live}}</ref>}} between the [[Military|armed forces]] of [[State (polity)|states]], or between [[government]]al forces and armed groups that are organized under a certain command structure and have the capacity to sustain [[military operation]]s, or between such organized groups.<ref>{{cite web |title=How is the Term "Armed Conflict" Defined in International Humanitarian Law? |url=https://www.icrc.org/sites/default/files/document_new/file_list/armed_conflict_defined_in_ihl.pdf |website=International Committee of the Red Cross |date=April 2024 |access-date=7 July 2024 |publisher=ICRC |pages=13–14 |archive-url=https://web.archive.org/web/20240708054327/https://www.icrc.org/sites/default/files/document_new/file_list/armed_conflict_defined_in_ihl.pdf |archive-date=8 July 2024}}</ref> 

It is generally characterized by widespread [[violence]], destruction, and mortality, using [[Regular army|regular]] or [[Irregular military|irregular]] [[Military|military forces]]. War aims typically involve the pursuit of political, economic, or territorial objectives. '''Warfare''' refers to the common activities and characteristics of types of war, or of wars in general.<ref>{{cite web|title=Warfare|url=http://dictionary.cambridge.org/us/dictionary/english/warfare|website=Cambridge Dictionary|access-date=1 August 2016|archive-date=24 February 2021|archive-url=https://web.archive.org/web/20210224072805/https://dictionary.cambridge.org/us/dictionary/english/warfare|url-status=live}}</ref> [[Total war]] is warfare that is not restricted to purely [[legitimate military target]]s, and can result in massive [[Civilian casualty|civilian]] or other [[non-combatant]] suffering and [[Casualty (person)|casualties]].

War has changed in numerous ways over the course of history. Since 1945,{{refn|group=lower-alpha|The final year of [[World War II]]}} great power wars, [[Conquest|territorial conquests]] and [[Declaration of war|war declarations]] have [[Long Peace|declined in frequency]].<ref name=":4" /> However, war in general has not necessarily declined.<ref name=":4" /> [[Civil war|Civil wars]] have increased in absolute terms since 1945.<ref name=":4" /> Wars have been increasingly regulated by [[international humanitarian law]].<ref name=":4" /> Battle deaths and casualties have declined, in part due to advances in military medicine.<ref name=":1" />  {{TOC limit|3}}

==Etymology==
The English word ''war'' derives from the 11th-century [[Old English]] words {{lang|ang|wyrre}} and {{lang|ang|werre}}, from [[Old French]] {{lang|fro|werre}} ({{lang|fr|guerre}} as in modern French), in turn from the [[Frankish language|Frankish]] {{lang|frk|*werra}}, ultimately deriving from the [[Proto-Germanic language|Proto-Germanic]] {{lang|gem-x-proto|*werzō}} {{gloss|mixture, confusion}}. The word is related to the [[Old Saxon]] {{lang|osx|werran}}, [[Old High German]] {{lang|goh|werran}}, and the modern German {{lang|de|verwirren}}, meaning {{gloss|to confuse, to perplex, to bring into confusion}}.<ref>{{cite dictionary |title=war |dictionary=Online Etymology Dictionary |url=http://www.etymonline.com/index.php?term=war |year=2010 |access-date=24 April 2011 |archive-date=11 January 2012 |archive-url=https://web.archive.org/web/20120111153016/http://www.etymonline.com/index.php?term=war |url-status=live }}</ref>

==History==
{{Main|Military history}}

[[File:Morella (combate-de-arquero.png|thumb|left|Cave painting in Galeria del Roure, in [[Morella, Spain|Morella la Vella]] depicting [[archery]] [[prehistoric warfare|combat]]<ref>Keith F. Otterbein, ''How War Began'' (2004), [https://books.google.com/books?id=4-sPSyNWVsEC&pg=PA72 p. 72].</ref>]]Anthropologists disagree about whether warfare was common throughout human prehistory, or whether it was a more recent development, following the invention of agriculture or organized states.<ref name=":3">{{Cite journal |last=Gat |first=Azar |author-link=Azar Gat |date=2015-05-06 |title=Proving communal warfare among hunter-gatherers: The quasi-rousseauan error |url=https://onlinelibrary.wiley.com/doi/10.1002/evan.21446 |journal=Evolutionary Anthropology: Issues, News, and Reviews |language=en |volume=24 |issue=3 |pages=111–126 |doi=10.1002/evan.21446 |pmid=26081116 |issn=1060-1538}}</ref><ref>{{Cite journal |last1=Allen |first1=Mark W. |last2=Bettinger |first2=Robert Lawrence |last3=Codding |first3=Brian F. |last4=Jones |first4=Terry L. |last5=Schwitalla |first5=Al W. |date=2016-10-25 |title=Resource scarcity drives lethal aggression among prehistoric hunter-gatherers in central California |journal=Proceedings of the National Academy of Sciences |language=en |volume=113 |issue=43 |pages=12120–12125 |doi=10.1073/pnas.1607996113 |doi-access=free |issn=0027-8424 |pmc=5087046 |pmid=27790997|bibcode=2016PNAS..11312120A }}</ref><ref>{{Cite book |last1=Haas |first1=Jonathan |chapter=The Prehistory of Warfare: Misled by Ethnography |date=2013-04-12 |title=War, Peace, and Human Nature |pages=168–190 |editor-last=Fry |editor-first=Douglas P. |chapter-url=https://academic.oup.com/book/12748/chapter/162858373 |access-date=2024-12-22 |edition=1st |publisher=Oxford University Press |language=en |doi=10.1093/acprof:oso/9780199858996.003.0010 |isbn=978-0-19-985899-6 |last2=Piscitelli |first2=Matthew|chapter-url-access=subscription |location=New York}}</ref><ref>{{Cite journal |last1=Kissel |first1=Marc |last2=Kim |first2=Nam C. |date=January 2019 |title=The emergence of human warfare: Current perspectives |url=https://onlinelibrary.wiley.com/doi/10.1002/ajpa.23751 |journal=American Journal of Physical Anthropology |language=en |volume=168 |issue=S67 |pages=141–163 |doi=10.1002/ajpa.23751 |pmid=30575025 |bibcode=2019AJPA..168S.141K |issn=0002-9483|url-access=subscription }}</ref><ref>{{Cite journal |last=Meijer |first=Hugo |date=September 2024 |title=The Origins of War: A Global Archaeological Review |url=https://link.springer.com/10.1007/s12110-024-09477-3 |journal=Human Nature |language=en |volume=35 |issue=3 |pages=225–288 |doi=10.1007/s12110-024-09477-3 |pmid=39638956 |issn=1045-6767|url-access=subscription }}</ref> It is difficult to determine whether warfare occurred during the [[Paleolithic]] due to the sparsity of known remains. Some sources claim that most Middle and Upper Paleolithic societies were possibly fundamentally [[egalitarianism|egalitarian]]<ref>{{cite book|author=McClellan |title=Science and Technology in World History: An Introduction |location=Baltimore |publisher=[[Johns Hopkins University Press]] |year=2006 |isbn=978-0-8018-8360-6 |url=https://books.google.com/books?id=aJgp94zNwNQC |pages=6–12}}</ref><ref>{{cite book |first=Leften Stavros |last=Stavrianos |url=https://books.google.com/books?id=MKhe6qNva10C&q=paleolithic+society |title=A Global History from Prehistory to the Present |location=New Jersey |publisher=[[Prentice Hall]] |year=1991 |isbn=978-0-13-357005-2 |pages=[https://books.google.com/books?id=MKhe6qNva10C&q=paleolithic+society 9–13]}}</ref><ref>{{cite book |title=Anthropology |last1=Miller |first1=Barbra |first2=Bernard |last2=Wood |first3=Andrew |last3=Balansky |first4=Julio |last4=Mercader |first5=Melissa |last5=Panger |year=2006 |publisher=Allyn and Bacon |location=Boston |isbn=978-0-205-32024-0 |page=768}}</ref><ref>Christopher Boehm (1999) [https://books.google.com/books?id=ljxS8gUlgqgC&dq=Paleolithic&pg=PA197 "Hierarchy in the Forest: The Evolution of Egalitarian Behavior" p. 198] Harvard University Press.</ref> and may have rarely or never engaged in organized violence between groups (i.e. war).<ref>{{cite book |title=Anthropology |last1=Miller |first1=Barbra |first2=Bernard |last2=Wood |first3=Andrew |last3=Balansky |first4=Julio |last4=Mercader |first5=Melissa |last5=Panger |year=2006 |publisher=Allyn and Bacon |location=Boston |isbn=978-0-205-32024-0 |page=768}}</ref><ref>{{cite book |first=R. Dale |last=Gutrie |url=https://books.google.com/books?id=3u6JNwMyMCEC&q=Paleolithic+religions&pg=PA428 |title=The Nature of Paleolithic art |location=Chicago |publisher=[[University of Chicago Press]] |year=2005 |isbn=978-0-226-31126-5}} [https://books.google.com/books?id=3u6JNwMyMCEC&dq=Paleolithic+religions&pg=PA428 pp. 420-22].</ref><ref name="Barbara Ehrenreich">{{cite book |first=Barbara |last=Ehrenreich |url=https://books.google.com/books?id=nFuDltu509YC |title=Blood Rites: Origins and History of the Passions of War |location=London |publisher=Macmillan |year=1997 |isbn=978-0-8050-5787-4}} [https://books.google.com/books?id=nFuDltu509YC p. 123].</ref><ref name="Kelly">{{cite journal |last=Kelly |first=Raymond |title=The evolution of lethal intergroup violence |doi=10.1073/pnas.0505955102 |journal=[[PNAS]]|volume=102 |date=October 2005 |pmid=16129826 |issue=43 |pmc=1266108 |pages=15294–98 |bibcode=2005PNAS..10215294K |doi-access=free}}</ref> Evidence of violent conflict appears to increase during the [[Mesolithic]] period, from around 10,000 years ago onwards.<ref name=":3" />

American cultural anthropologist and ethnologist [[Raymond C. Kelly]] claimed that before 400,000 years ago, humans clashed like groups of chimpanzees; however, later they preferred "positive and peaceful social relations between neighboring groups, such as joint hunting, trading, and courtship".<ref>{{cite journal |last1=Zagorski |first1=Nick |title=Profile of Raymond C. Kelly |journal=Proceedings of the National Academy of Sciences |date=2005 |volume=102 |issue=51 |pages=18249–18251 |publisher=National Academy of Sciences |doi=10.1073/pnas.0506968102 |doi-access=free |pmid=16352723 |pmc=1317930 |bibcode=2005PNAS..10218249Z }}</ref> In his book ''Warless Societies and the Origin of War'' he explores the origins of modern wars and states that high surplus product encourages conflict, so "raiding often begins in the richest environments".<ref>{{cite journal |last1=Flannery |first1=Kent V. |last2=Marcus |first2=Joyce |title=The origin of war: New 14C dates from ancient Mexico |journal=Proceedings of the National Academy of Sciences of the United States of America |date=2003 |volume=100 |issue=20 |pages=11801–11805 |doi=10.1073/pnas.1934526100 |doi-access=free |pmid=14500785 |pmc=208841 }}</ref>  

In his 1996 book ''[[War Before Civilization]]'', [[Lawrence H. Keeley]], a professor at the [[University of Illinois at Urbana–Champaign|University of Illinois]], says approximately 90–95% of known societies throughout history engaged in at least occasional warfare,<ref>{{cite web |url=http://brneurosci.org/reviews/war.html |title=Review: War Before Civilization |publisher=Brneurosci.org |date=4 September 2006 |access-date=2011-01-24 |url-status=usurped |archive-url=https://web.archive.org/web/20101121021158/http://brneurosci.org/reviews/war.html |archive-date=21 November 2010}}</ref> and many fought constantly.<ref>{{cite news|url=http://www.atimes.com/atimes/Front_Page/HG04Aa02.html |archive-url=https://web.archive.org/web/20060706042537/http://www.atimes.com/atimes/Front_Page/HG04Aa02.html|url-status=unfit|archive-date=6 July 2006|title=The fraud of primitive authenticity|last=Spengler|date=4 July 2006|work=Asia Times Online|access-date=2009-06-08}}</ref> Keeley describes several styles of primitive combat such as small [[Raid (military)|raid]]s, large raids, and [[massacre]]s. All of these forms of warfare were used by primitive societies, a finding supported by other researchers.<ref>{{cite book |editor-last1=Martin |editor-first1=Debra L. |editor-first2=Ryan P. |editor-last2=Harrod |editor-first3=Ventura R. |editor-last3=Pérez |date=2012 |title=The Bioarchaeology of Violence |location=Gainesville |publisher=University Press of Florida |url=http://www.upf.com/book.asp?id=MARTI002 |access-date=10 January 2013 |archive-date=4 November 2013 |archive-url=https://web.archive.org/web/20131104230028/http://upf.com/book.asp?id=MARTI002 |url-status=live }}</ref> Keeley explains that early war raids were not well organized, as the participants did not have any formal training. Scarcity of resources meant [[defensive works]] were not a cost-effective way to protect the society against enemy raids.<ref name="Keeley page 55">Keeley, Lawrence H. ''War Before Civilization: The Myth of the Peaceful Savage''. p. 55.</ref> [[William Rubinstein]] wrote "Pre-literate societies, even those organized in a relatively advanced way, were renowned for their studied cruelty.'"<ref>{{cite book|author=W. D. Rubinstein|title=Genocide: A History|url=https://books.google.com/books?id=nMMAk4VwLLwC&pg=PA22|access-date=31 May 2012|year=2004|publisher=Pearson Longman|isbn=978-0-582-50601-5|pages=22|archive-date=8 August 2013|archive-url=https://web.archive.org/web/20130808075142/http://books.google.com/books?id=nMMAk4VwLLwC&pg=PA22|url-status=live}}</ref> 

Since the rise of the state some 5,000 years ago,<ref>Diamond, Jared, ''Guns, Germs and Steel''.</ref> military activity has continued over much of the globe. In Europe the oldest known battlefield is thought to date to 1250 BC.<ref>{{cite news |last1=Strickland |first1=Ashley |title=Thousands of bones and hundreds of weapons reveal grisly insights into a 3,250-year-old battle |url=https://edition.cnn.com/2024/09/23/science/tollense-valley-bronze-age-battlefield-arrowheads/index.html |access-date=6 February 2025 |agency=CNN |date=24 September 2024}}</ref> The [[Bronze Age]] has been described as a key period in the intensification of warfare, with the emergence of dedicated warriors and the development of metal weapons like swords.<ref>{{Cite book |last1=Horn |first1=Christian |chapter=Introducing Bronze Age Warfare |date=2018-04-26 |title=Warfare in Bronze Age Society |pages=1–15 |editor-last=Horn |editor-first=Christian |chapter-url=https://www.cambridge.org/core/product/identifier/9781316884522%23CN-bp-1/type/book_part |access-date=2024-12-22 |edition=1 |publisher=Cambridge University Press |doi=10.1017/9781316884522.002 |isbn=978-1-316-88452-2 |last2=Kristiansen |first2=Kristian |editor2-last=Kristiansen |editor2-first=Kristian|chapter-url-access=subscription }}</ref> Two other commonly named periods of increase are the [[Axial Age]] and Modern Times.<ref>{{interlanguage link|Jack Levy|lt=Levy, Jack|de|Jack Levy}} & {{interlanguage link|William R. Thompson (political scientist)|lt=Thompson, William R.|de|William R. Thompson}} (2011). ''The Arc of War: Origins, Escalation, and Transformation'', (University of Chicago Press).</ref> The invention of [[gunpowder]], and its eventual use in warfare, together with the acceleration of technological advances, have fomented major changes to war itself.

[[File:War deaths caused by warfare.svg|thumb|right|upright=1.4|The percentages of men killed in war in eight tribal societies, and Europe and the U.S. in the 20th century. (Lawrence H. Keeley, archeologist)]]

In ''[[Coercion, Capital, and European States, AD 990–1992]]'', [[Charles Tilly]] argued that "war made the state, and the state made war", saying that wars have led to creation of states which in their turn perpetuate war.<ref>{{cite journal |last1=Young |first1=Laura D. |title=Testing Tilly: Does War Really Make States? |journal=Social Evolution & History |date=March 2022 |volume=21 |doi=10.30884/seh/2022.01.07 |url=https://www.sociostudies.org/journal/articles/3275058/ |access-date=4 February 2025|doi-access=free }}</ref><ref>{{cite web |title=War Did Make States: Testing Tilly's Thesis |url=https://istp.ethz.ch/events/colloquia/reports/2020/war-did-make-states-testing-tillys-thesis.html |website=Institute of Science, Technology and Policy |publisher=Eidgenössische Technische Hochschule Zürich |access-date=4 February 2025}}</ref> Tilly's theory of state formation is considered dominant in the state formation literature.<ref name=":7">{{Cite book |last1=Gorski |first1=Philip |chapter=Beyond the Tilly Thesis: "Family Values" and State Formation in Latin Christendom |date=2017 |chapter-url=https://www.cambridge.org/core/books/does-war-make-states/beyond-the-tilly-thesis/4D74B3D06884F763CA76307BABFF798B |title=Does War Make States?: Investigations of Charles Tilly's Historical Sociology |pages=98–124 |editor-last=Strandsbjerg |editor-first=Jeppe |publisher=Cambridge University Press |isbn=978-1-107-14150-6 |last2=Sharma |first2=Vivek Swaroop |editor2-last=Kaspersen |editor2-first=Lars Bo}}</ref><ref>{{Cite book|last=Ertman|first=Thomas|url=https://books.google.com/books?id=dQ53vjKiwR0C|title=Birth of the Leviathan: Building States and Regimes in Medieval and Early Modern Europe|date=1997|publisher=Cambridge University Press|isbn=978-0-521-48427-5|pages=4|language=en}}</ref><ref>{{Cite book|last=Bagge|first=Sverre|url=https://books.google.com/books?id=NFJNAgAAQBAJ|title=Cross and Scepter: The Rise of the Scandinavian Kingdoms from the Vikings to the Reformation|date=2014|publisher=Princeton University Press|isbn=978-1-4008-5010-5|pages=4|language=en}}</ref>

Since 1945, great power wars, [[Conquest|territorial conquests]] and [[Declaration of war|war declarations]] have [[Long Peace|declined in frequency]].<ref name=":4">{{Cite journal |last=Fazal |first=Tanisha M. |date=2025 |title=Is War in Decline? |journal=Annual Review of Political Science |volume=28 |pages=57–73 |language=en |doi=10.1146/annurev-polisci-041923-115351|doi-access=free }}</ref> Wars have been increasingly regulated by [[international humanitarian law]].<ref name=":4" /> Battle deaths and casualties have declined, in part due to advances in military medicine<ref name=":1">{{Cite journal |last=Fazal |first=Tanisha M. |date=2014 |title=Dead Wrong? Battle Deaths, Military Medicine, and Exaggerated Reports of War's Demise |url=https://www.jstor.org/stable/24480546 |journal=International Security |volume=39 |issue=1 |pages=95–125 |doi=10.1162/ISEC_a_00166 |jstor=24480546 |issn=0162-2889|url-access=subscription }}</ref> and despite advances in weapons. In Western Europe, since the late 18th century, more than 150 conflicts and about 600 battles have taken place, but no battle has taken place since 1945.<ref name="War">{{usurped|1=[https://web.archive.org/web/20180227114356/http://www.ralphmag.org/CG/world-war-one2.html World War One – A New Kind of War {{!}} Part II]}}, From ''14–18 Understanding the Great War'', by Stéphane Audoin-Rouzeau, Annette Becker.</ref> According to [[Michael Spagat]] and William Thompson, great power war in Western Europe shows a sustained decline since the 16th century.<ref>{{Cite journal |last1=Spagat |first1=Michael |last2=Thompson |first2=William |date=2026 |title=Great Power Warfare and the Decline of War Question |url=https://doi.org/10.1093/isq/sqag046 |journal=International Studies Quarterly |volume=70 |issue=3 |article-number=sqag046 |doi=10.1093/isq/sqag046 |issn=0020-8833}}</ref>

Scholars have debated the question of decline in wars and the purported causes behind such a decline. [[Joshua S. Goldstein|Joshua Goldstein]], [[John Mueller]] and [[Steven Pinker]] have argued that war is in decline, while [[Bear F. Braumoeller]] has challenged such claims.<ref>{{Cite journal |last=Duerr |first=Benjamin |date=2020-05-01 |title=Only the dead: the persistence of war in the modern age |url=https://academic.oup.com/ia/article/96/3/821/5831454 |journal=International Affairs |language=en |volume=96 |issue=3 |pages=821–822 |doi=10.1093/ia/iiaa069 |issn=0020-5850}}</ref> 

However, [[Civil war|civil wars]] have increased in absolute terms since 1945.<ref name=":4" /> A distinctive feature of war since 1945 is that combat has largely been a matter of civil wars and insurgencies.<ref>Robert J. Bunker and Pamela Ligouri Bunker, "The modern state in epochal transition: The significance of irregular warfare, state deconstruction, and the rise of new warfighting entities beyond neo-medievalism." ''Small Wars & Insurgencies'' 27.2 (2016): 325–344.</ref> The major exceptions were the [[Korean War]], the [[Indo-Pakistani War of 1971]], the [[Iran–Iraq War]], the [[Gulf War]], the [[Eritrean–Ethiopian War]], and the [[Russo-Ukrainian War]].

==Types of warfare==
<!-- include "warfare" in heading to denote section's focus on methods of conducting wars, excluding their causes -->
{{Main|Outline of war#Types of war|l1 = Types of war}}

* [[Asymmetric warfare]] is the methods used in conflicts between [[belligerent]]s of drastically different levels of military capability or size.<ref>{{Cite web |title=Asymmetrical warfare {{!}} Britannica |url=https://www.britannica.com/topic/asymmetrical-warfare |access-date=2023-05-05 |website=www.britannica.com |language=en}}</ref>
* [[Biological warfare]], or germ warfare, is the use of biological infectious agents or toxins such as [[bacteria]], [[Virus|viruses]], and [[Fungus|fungi]] against people, plants, or animals. This can be conducted through sophisticated technologies, like [[Cluster munition|cluster munitions]],<ref>{{Cite journal |last=Guillemin |first=Jeanne |date=July 2006 |title=Scientists and the history of biological weapons: A brief historical overview of the development of biological weapons in the twentieth century |journal=EMBO Reports |language=en |volume=7 |issue=S1 |pages=S45-9 |doi=10.1038/sj.embor.7400689 |issn=1469-221X |pmc=1490304 |pmid=16819450}}</ref> or with rudimentary techniques like catapulting an infected corpse behind enemy lines,<ref>{{Cite journal |last=Wheelis |first=Mark |title=Biological Warfare at the 1346 Siege of Caffa - Volume 8, Number 9—September 2002 - Emerging Infectious Diseases journal - CDC |journal=Emerging Infectious Diseases |year=2002 |volume=8 |issue=9 |pages=971–975 |url=https://wwwnc.cdc.gov/eid/article/8/9/01-0536_article |language=en-us |doi=10.3201/eid0809.010536|pmid=12194776 |pmc=2732530  |issn=1080-6040}}</ref> and can include weaponized or non-weaponized pathogens.
* [[Chemical warfare]] involves the use of weaponized chemicals in combat. Poison gas as a [[Chemical warfare|chemical weapon]] was principally used during [[World War I]], and resulted in over a million estimated casualties, including more than 100,000 civilians.<ref>{{cite book |title=Handbook of Chemical and Biological Warfare Agents |edition=2nd |author=D. Hank Ellison |date= 2007 |pages=567–570 |publisher=[[CRC Press]] |isbn=978-0-8493-1434-6}}</ref>
* [[Cold warfare]] is an intense international rivalry without direct military conflict, but with a sustained threat of it, including high levels of military preparations, expenditures, and development, and may involve active conflicts by indirect means, such as [[economic warfare]], [[political warfare]], [[covert operation]]s, [[espionage]], [[cyberwarfare]], or [[proxy war]]s.
* [[Conventional warfare]] is a form of warfare between states in which [[nuclear weapon|nuclear]], [[biological weapon|biological]], [[chemical weapons|chemical]] or [[Radiological warfare|radiological weapons]] are not used or see limited deployment.
* [[Cyberwarfare]] involves the actions by a nation-state or international organization to attack and attempt to damage another nation's information systems.
* [[Insurgency]] is a rebellion against authority, where irregular forces take up arms to change an existing political order. An insurgency can be fought via [[counterinsurgency]], and may also be opposed by measures to protect the population, and by political and economic actions of various kinds aimed at undermining the insurgents' claims against the incumbent regime.
* [[Information warfare]] is the application of destructive force on a large scale against information assets and systems, against the [[computer]]s and [[Computer network|networks]] that support the four critical infrastructures (the power grid, communications, financial, and transportation).<ref>{{Cite web|url=https://fas.org/irp/eprint/snyder/infowarfare.htm |title=Information Warfare |last=Lewis |first=Brian C. |website=Federation of American Scientist |archive-url=https://web.archive.org/web/19970617035106/http://www.fas.org/irp/eprint/snyder/infowarfare.htm |archive-date=17 June 1997 |url-status=dead |access-date=27 February 2017 }}</ref>
* [[Nuclear warfare]] is warfare in which [[nuclear weapon]]s are the primary, or a major, method of achieving capitulation.
* [[Radiological warfare]] is any form of warfare involving deliberate [[Radioactive contamination|radiation poisoning or contamination]] of an area with radiological sources.
* [[Total war]] is warfare by any means possible, disregarding the [[laws of war]], placing no limits on [[legitimate military target]]s, using [[weapon]]s and [[Military tactics|tactics]] resulting in significant [[civilian casualties]], or demanding a [[war effort]] requiring significant sacrifices by the friendly civilian population.
* [[Unconventional warfare]] can be defined as "military and quasi-military operations other than [[conventional warfare]]"<ref name="nagao definition">{{cite web |last1=Nagao |first1=Yuichiro |title=Unconventional Warfare: A Historical Perspective |url=http://www.nids.mod.go.jp/english/event/symposium/pdf/2001/sympo_e2001_6.pdf |website=National Institute for Defense Studies |publisher=[[Ministry of Defense (Japan)|Ministry of Defense]] |access-date=18 August 2022 |archive-url=https://web.archive.org/web/20220816015847/http://www.nids.mod.go.jp/english/event/symposium/pdf/2001/sympo_e2001_6.pdf |archive-date=16 August 2022 |date=2001}}</ref> and may use [[covert operation|covert]] forces or actions such as [[subversion]], [[Decoy|diversion]], [[sabotage]], [[espionage]], [[biowarfare]], [[economic sanctions|sanctions]], [[propaganda]] or [[guerrilla warfare]].

==Aims==
[[File:Flickr - DVIDSHUB - Firefight in the Waterpur Valley.jpg|thumb|[[United States Army]] soldiers engaged in a firefight with [[Taliban insurgency|Taliban insurgents]] during the [[War in Afghanistan (2001–2021)|War in Afghanistan]], 2009]]

Entities contemplating going to war and entities considering whether to end a war may formulate ''war aims'' as an evaluation/propaganda tool. War aims may stand as a proxy for national-military resolve.<ref>{{cite book
 |last1            = Sullivan
 |first1           = Patricia
 |title            = Who Wins?: Predicting Strategic Success and Failure in Armed Conflict
 |chapter-url      = https://books.google.com/books?id=ceushfFUCpUC
 |publisher        = Oxford University Press, US
 |page             = 17
 |publication-date = 2012
 |doi              = 10.1093/acprof:oso/9780199878338.003.0003
 |isbn             = 978-0199878338
 |access-date      = 2015-08-25
 |quote            = A state with greater military capacity than its adversary is more likely to prevail in wars with 'total' war aims{{snd}}the overthrow of a foreign government or annexation of territory{{snd}}than in wars with more limited objectives.
 |date             = 2012
 |chapter          = War Aims and War Outcomes
 |archive-date     = 13 September 2015
 |archive-url      = https://web.archive.org/web/20150913023410/https://books.google.com/books?id=ceushfFUCpUC
 |url-status       = live
}}</ref>

=== Definition ===
Fried defines war aims as "the desired territorial, economic, military or other benefits expected following successful conclusion of a war".<ref>{{cite book
 |last1            = Fried
 |first1           = Marvin Benjamin
 |title            = Austro-Hungarian War Aims in the Balkans During World War I
 |url              = https://books.google.com/books?id=ETDtAwAAQBAJ
 |publisher        = Palgrave Macmillan
 |publication-date = 2014
 |page             = 4
 |isbn             = 978-1137359018
 |access-date      = 2015-08-24
 |quote            = War aims are the desired territorial, economic, military or other benefits expected following successful conclusion of a war.
 |date             = 2014-07-01
 |archive-date     = 17 October 2015
 |archive-url      = https://web.archive.org/web/20151017214443/https://books.google.com/books?id=ETDtAwAAQBAJ
 |url-status       = live
}}</ref>

=== Classification ===

Tangible/intangible aims:
* Tangible war aims may involve (for example) the acquisition of territory (as in the German goal of [[Lebensraum]] in the first half of the 20th century) or the recognition of economic concessions (as in the [[Anglo-Dutch Wars]]).
* Intangible war aims – like the accumulation of credibility or reputation<ref>Welch distinguishes: "tangible goods such as arms, wealth, and – provided they are strategically or economically valuable – territory and resources" from "intangible goods such as credibility and reputation" – {{cite book
 |last1            = Welch
 |first1           = David A.
 |title            = Justice and the Genesis of War
 |url              = https://books.google.com/books?id=i2Z5blE1KGoC
 |series           = Cambridge Studies in International Relations
 |publisher        = Cambridge University Press
 |publication-date = 1995
 |page             = 17
 |isbn             = 978-0521558686
 |access-date      = 2015-08-24
 |date             = 1995
 |archive-date     = 18 September 2015
 |archive-url      = https://web.archive.org/web/20150918185242/https://books.google.com/books?id=i2Z5blE1KGoC
 |url-status       = live
}}</ref> – may have more tangible expression ("conquest restores prestige, annexation increases power").<ref>{{cite book
 |last1            = Fried
 |first1           = Marvin Benjamin
 |title            = Austro-Hungarian War Aims in the Balkans During World War I
 |url              = https://books.google.com/books?id=ETDtAwAAQBAJ
 |publisher        = Palgrave Macmillan
 |publication-date = 2014
 |page             = 4
 |isbn             = 978-1137359018
 |access-date      = 2015-08-24
 |quote            = Intangibles, such as prestige or power, can also represent war aims, though often (albeit not always) their achievement is framed within a more tangible context (e.g. conquest restores prestige, annexation increases power, etc.).
 |date             = 2014-07-01
 |archive-date     = 17 October 2015
 |archive-url      = https://web.archive.org/web/20151017214443/https://books.google.com/books?id=ETDtAwAAQBAJ
 |url-status       = live
}}</ref>

Explicit/implicit aims:
* Explicit war aims may involve published policy decisions.
* Implicit war aims<ref>Compare:{{cite news
 |last1        = Katwala
 |first1       = Sunder
 |author-link1 = Sunder Katwala
 |title        = Churchill by Paul Addison
 |url          = https://www.theguardian.com/books/2005/feb/13/biography.features1
 |department   = Books
 |newspaper    = The Guardian
 |publisher    = Guardian News and Media Limited
 |date         = 2005-02-13
 |access-date  = 2015-08-24
 |quote        = [Churchill] took office and declared he had 'not become the King's First Minister to oversee the liquidation of the British empire'. [...] His view was that an Anglo-American English-speaking alliance would seek to preserve the empire, though ending it was among Roosevelt's implicit war aims.
 |archive-date = 28 September 2016
 |archive-url  = https://web.archive.org/web/20160928012352/https://www.theguardian.com/books/2005/feb/13/biography.features1
 |url-status   = live
}}</ref> can take the form of minutes of discussion, memoranda and instructions.<ref>Compare {{cite book
 |last1            = Fried
 |first1           = Marvin Benjamin
 |title            = Austro-Hungarian War Aims in the Balkans During World War I
 |url              = https://books.google.com/books?id=ETDtAwAAQBAJ
 |publisher        = Palgrave Macmillan
 |publication-date = 2014
 |page             = 4
 |isbn             = 978-1137359018
 |access-date      = 2015-08-24
 |quote            = At times, war aims were explicitly stated internally or externally in a policy decision, while at other times [...] the war aims were merely discussed but not published, remaining instead in the form of memoranda or instructions.
 |date             = 2014-07-01
 |archive-date     = 17 October 2015
 |archive-url      = https://web.archive.org/web/20151017214443/https://books.google.com/books?id=ETDtAwAAQBAJ
 |url-status       = live
}}</ref>

Positive/negative aims:
* "Positive war aims" cover tangible outcomes.
* "Negative war aims" forestall or prevent undesired outcomes.<ref>{{cite book
 |last1            = Fried
 |first1           = Marvin Benjamin
 |chapter          = 'A Life and Death Question': Austro-Hungarian War Aims in the First World War
 |editor1-last     = Afflerbach
 |editor1-first    = Holger
 |title            = The Purpose of the First World War: War Aims and Military Strategies
 |chapter-url      = https://books.google.com/books?id=A6AFCgAAQBAJ
 |series           = Schriften des Historischen Kollegs
 |volume           = 91
 |location         = Berlin/Boston
 |publisher        = Walter de Gruyter GmbH
 |publication-date = 2015
 |page             = 118
 |isbn             = 978-3110443486
 |access-date      = 2015-08-24
 |quote            = [T]he [Austrian] Foreign Ministry [...] and the Military High Command [...] were in agreement that political and military hegemony over Serbia and the Western Balkans was a vital war aim. The Hungarian Prime Minister István Count Tisza, by contrast, was more preoccupied with so-called 'negative war aims', notably warding off hostile Romanian, Italian, and even Bulgarian intervention.
 |date             = 2015-07-01
 |archive-date     = 16 October 2015
 |archive-url      = https://web.archive.org/web/20151016235519/https://books.google.com/books?id=A6AFCgAAQBAJ
 |url-status       = live
}}</ref>

War aims can change in the course of conflict and may eventually morph into "peace conditions"<ref>{{cite book
 |last1            = Haase
 |first1           = Hugo
 |author-link1     = Hugo Haase
 |chapter          = The Debate in the Reichstag on Internal Political Conditions, April 5–6, 1916
 |editor1-last     = Lutz
 |editor1-first    = Ralph Haswell
 |title            = Fall of the German Empire, 1914–1918
 |chapter-url      = https://books.google.com/books?id=cW6mAAAAIAAJ
 |series           = Hoover War Library publications
 |publisher        = Stanford University Press
 |publication-date = 1932
 |page             = 233
 |isbn             = 978-0804723800
 |access-date      = 2015-08-25
 |quote            = Gentlemen, when it comes time to formulate peace conditions, it is time to think of another thing than war aims.
 |year             = 1932
 |archive-date     = 25 October 2015
 |archive-url      = https://web.archive.org/web/20151025051415/https://books.google.com/books?id=cW6mAAAAIAAJ
 |url-status       = live
}}</ref> – the minimal conditions under which a state may cease to wage a particular war.

==Effects==
[[File:Wars-Long-Run-military-civilian-fatalities.png|thumb|upright=1.35|Global deaths in conflicts since the year 1400<ref>{{Cite journal|url=https://ourworldindata.org/war-and-peace/|title=War and Peace|last=Roser|first=Max|date=2017-11-15|journal=Our World in Data|access-date=2017-11-15|archive-date=16 November 2017|archive-url=https://web.archive.org/web/20171116083027/https://ourworldindata.org/war-and-peace/|url-status=live}}</ref>]]{{Main|Effects of war}}

===Conflict zones===
When a war takes place, one or more areas within a country or across border becomes a war zone or conflict zone. Daily life is interrupted, travel to or across the area may be difficult and international visitors may be advised to leave the area.<ref>[[Government of Canada]], [https://www.international.gc.ca/world-monde/stories-histoires/2024/2024-11-13-conflict-zone-conflit.aspx?lang=eng Travelling to or living in a conflict zone? Read on.], updated on 18 November 2024, accessed on 18 April 2025.</ref>

===Casualties ===
[[File:War world map - DALY - WHO2004.svg|thumb|upright=1.35|[[Disability-adjusted life year]] for war per 100,000&nbsp;inhabitants in 2004<ref>{{cite web |url=https://www.who.int/entity/healthinfo/global_burden_disease/gbddeathdalycountryestimates2004.xls |title=Mortality and Burden of Disease Estimates for WHO Member States in 2004 |publisher=World Health Organization |access-date=5 October 2020 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828123901/https://www.who.int/healthinfo/global_burden_disease/gbddeathdalycountryestimates2004.xls |url-status=live }}</ref>{{Div col|small=yes|colwidth=10em}}
{{legend|#b3b3b3|no data}}
{{legend|#ffff65|less than 100}}
{{legend|#fff200|100–200}}
{{legend|#ffdc00|200–600}}
{{legend|#ffc600|600–1000}}
{{legend|#ffb000|1000–1400}}
{{legend|#ff9a00|1400–1800}}
{{legend|#ff8400|1800–2200}}
{{legend|#ff6e00|2200–2600}}
{{legend|#ff5800|2600–3000}}
{{legend|#ff4200|3000–8000}}
{{legend|#ff2c00|8000–8800}}
{{legend|#cb0000|more than 8800}}
{{div col end}}]]

Estimates for total deaths due to war vary widely. In one estimate, primitive warfare from 50,000 to 3000 BCE has been thought to have claimed 400{{nbsp}}million±133,000 victims based on the assumption that it accounted for the 15.1% of all deaths.<ref>Matthew White, [http://necrometrics.com/pre1700b.htm#Primitive 'Primitive War'] {{Webarchive|url=https://web.archive.org/web/20130414061247/http://necrometrics.com/pre1700b.htm#Primitive |date=14 April 2013 }}.</ref> [[Ian Morris (historian)|Ian Morris]] estimated that the rate could be as high as 20%.<ref>Morris, Ian (2012). "The evolution of war," ''Cliodynamics'', vol. 3 (1): p. 9, https://escholarship.org/uc/item/8jr9v920.</ref> Other scholars find the prehistoric percentage much lower, around 2%, similar to the Neanderthals and ancestors of apes and primates.<ref>Gómez, José María et al. (Summer 2016). "The phylogenetic roots of human lethal violence," ''Nature'', vol. 538 (7624), https://www.uv.es/~verducam/HHL.pdf.</ref> 

For the period 3000 BCE until 1991, estimates range from 151{{nbsp}}million<ref>{{cite journal |last1=Eckhardt |first1=William |date=1991 |title=War-related deaths since 3000 BC. |url= |journal=Bulletin of Peace Proposals |volume= 22|issue=4 |pages=437–443 |doi= 10.1177/096701069102200410|s2cid=144946896 |access-date=}} https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=27e7fdb7d9b671cdcf999f3aab15cca8be25b163.</ref> to several{{nbsp}}billion.<ref>[[Ian Morris (historian)|Morris, Ian]] (2012). "The evolution of war," ''Cliodynamics'', vol. 3 (1): p. 9-37, https://escholarship.org/uc/item/8jr9v920.</ref><ref>{{Cite book |last=Morris |first=Ian |url=https://books.google.com/books?id=FbxXAgAAQBAJ&q=per+cent |title=War! What Is It Good For?: Conflict and the Progress of Civilization from Primates to Robots |date=2014-04-15 |publisher=Macmillan + ORM |isbn=978-0-374-71103-0 |language=en}}</ref> The lowest estimate for history of 151 million was calculated by William Eckhardt. He explained his method as summing the recorded casualties and multiplying their average by the number of recorded battles or wars. This method excludes indirect deaths for premodern wars and all deaths for unrecorded wars. Few premodern wars were recorded beyond Eurasia and only 18 wars were recorded for period 3000–1500 BC worldwide.<ref>{{cite journal |last1=Eckhardt |first1=William |date=1991 |title=War-related deaths since 3000 BC. |url= |journal=Bulletin of Peace Proposals |volume= 22|issue=4 |pages=437–441 |doi= 10.1177/096701069102200410|s2cid=144946896 |access-date=}} https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=27e7fdb7d9b671cdcf999f3aab15cca8be25b163.</ref> 

Data cumulated since the research of Eckhardt, especially for the non-European world, was collected in this [[List of wars by death toll|list]]. Averaging the ranges it provided, the total for 500 BC–2023 AD is about 570 million, or 0.95% of people born in the same period, of them 58 million for 500 BC–AD 500, 117 million for AD 500–1700, and 396 million for 1700–2023.<ref>{{Cite web |last=McFadden |first=Christopher |title=10 weapons that have killed the most people since war began |url=https://interestingengineering.com/military/the-worlds-10-deadliest-weapons |access-date=2026-06-24 |website=Interesting Engineering |language=en}}</ref>

Meanwhile, researchers shifted from Eckhardt's approach to general estimations of the percentage of the population killed by wars. [[Azar Gat]] and [[Ian Morris (historian)|Ian Morris]] both give the lowest estimate of 1% for history including all the 20th century,<ref name="a gat">{{cite web |last=Gat |first=Azar |title=Is war declining – and why? |url=https://is.muni.cz/el/1423/podzim2015/MVZ208/um/Journal_of_Peace_Research-2013-Gat-149-57.pdf |website=is.muni.cz |date=2013 |access-date=June 25, 2026}}</ref>
<ref>[[Ian Morris (historian)|Morris, Ian]] (2012). "The evolution of war," ''Cliodynamics'', vol. 3 (1): p. 9-37, https://escholarship.org/uc/item/8jr9v920.</ref><ref>{{Cite book |last=Morris |first=Ian |url=https://books.google.com/books?id=FbxXAgAAQBAJ&q=per+cent |title=War! What Is It Good For?: Conflict and the Progress of Civilization from Primates to Robots |date=2014-04-15 |publisher=Macmillan + ORM |isbn=978-0-374-71103-0 |language=en}}</ref> or about 1 billion.<ref>{{Cite journal |last=Haub |first=C. |date=February 1995 |title=How many people have ever lived on earth? |journal=Population Today |volume=23 |issue=2 |pages=4–5 |issn=0749-2448 |pmid=12288594}}</ref> The highest estimates of both scholars exceed the famous "hoax" of 3,640,000,000 people killed in wars which circulated decades in scholarly literature in various countries.<ref>Jongman, B. & Dennen J. M. G. van der (2005). "The great 'war figures' hoax: An investigation in polemomythology," (University of Groningen), https://core.ac.uk/download/pdf/148292168.pdf.</ref> Gat gives 5%,<ref name="a gat" /> or about 5 billion.<ref>{{Cite web |date=2022-11-15 |title=How Many People Have Ever Lived on Earth? |url=https://www.prb.org/news/how-many-people-have-ever-lived-on-earth/ |access-date=2026-06-24 |website=Population Reference Bureau |language=en-US}}</ref> Morris gives for the 20th century 2%, for 1400–1900 3% in Europe and "slightly higher" elsewhere, 5% for the ancient empires in 500 BC–AD 200, 10% for the rest of history and 20% for prehistory.<ref>[[Ian Morris (historian)|Morris, Ian]] (2012). "The evolution of war," ''Cliodynamics'', vol. 3 (1): p. 9-37, https://escholarship.org/uc/item/8jr9v920.</ref><ref>{{Cite book |last=Morris |first=Ian |url=https://books.google.com/books?id=FbxXAgAAQBAJ&q=per+cent |title=War! What Is It Good For?: Conflict and the Progress of Civilization from Primates to Robots |date=2014-04-15 |publisher=Macmillan + ORM |isbn=978-0-374-71103-0 |language=en}}</ref> His total for history is thus about 9 billion.<ref>{{Cite web |date=2022-11-15 |title=How Many People Have Ever Lived on Earth? |url=https://www.prb.org/news/how-many-people-have-ever-lived-on-earth/ |access-date=2026-06-24 |website=Population Reference Bureau |language=en-US}}</ref>

===Largest wars by death toll===
{{Main|List of wars by death toll}}

The deadliest war in history, in terms of the cumulative number of deaths since its start, is [[World War II]], from 1939 to 1945, with 70–85&nbsp;million deaths, followed by the [[Mongol conquests]]<ref>*The Cambridge History of China: Alien regimes and border states, 907–1368, 1994, p. 622, cited by White.
<br />*Matthew White (2011). ''The Great Big Book of Horrible Things: The Definitive Chronicle of History's 100 Worst Atrocities''.</ref> at up to 60&nbsp;million. As concerns a belligerent's losses in proportion to its prewar population, the most destructive war in [[modern history]] may have been the [[Paraguayan War]] (see [[Paraguayan War casualties]]). War resulted in 31,000 deaths in 2013, down from 72,000 deaths in 1990.<ref name="GDB2013">{{cite journal|collaboration= | first1= Christopher JL| last1 = Murray|first2=Theo|last2= Vos|first3=Alan D.|last3= Lopez|title=Global, regional, and national age-sex specific all-cause and cause-specific mortality for 240 causes of death, 1990–2013: a systematic analysis for the Global Burden of Disease Study 2013|journal=Lancet|date=17 December 2014|pmid=25530442|doi=10.1016/S0140-6736(14)61682-2|pmc=4340604|volume=385|issue=9963|pages=117–71}}</ref> 

War usually results in significant deterioration of infrastructure and the ecosystem, a decrease in social spending, [[famine]], large-scale emigration from the war zone, and often the mistreatment of [[prisoner of war|prisoners of war]] or civilians.<ref>{{cite book|last1=Tanton|first1=John|title=The Social Contract|date=2002|page=42}}</ref><ref>{{cite book|last1=Moore|first1=John|title=The pursuit of happiness|date=1992|page=304}}</ref><ref>{{cite book|last1=Baxter|first1=Richard|title=Humanizing the Laws of War|date=2013|page=344}}</ref> For instance, of the nine million people who were on the territory of the [[Byelorussian Soviet Socialist Republic|Byelorussian SSR]] in 1941, some 1.6&nbsp;million were killed by the Germans in actions away from battlefields, including about 700,000 prisoners of war, 500,000 Jews, and 320,000 people counted as partisans (the vast majority of whom were unarmed civilians).<ref>Timothy Snyder, ''Bloodlands: Europe Between Hitler and Stalin'', Basic Books, 2010, p. 250.</ref> Another byproduct of some wars is the prevalence of [[propaganda]] by some or all parties in the conflict,<ref>''Dying and Death: Inter-disciplinary Perspectives''. p. 153, Asa Kasher (2007).</ref> and increased revenues by [[weapons manufacturers]].<ref>{{cite book|last1=Chew|first1=Emry|title=Arming the Periphery|date=2012|page=49}}</ref>

Three of the ten most costly wars, in terms of loss of life, have been waged in the last century. These are the two World Wars, followed by the [[Second Sino-Japanese War]] (which is sometimes considered part of [[World War II]], or as overlapping). Most of the others involved China or neighboring peoples. The death toll of World War II, being over 60&nbsp;million, surpasses all other war-death-tolls.<ref name="users.erols">McFarlane, Alan: ''The Savage Wars of Peace: England, Japan and the Malthusian Trap'', Blackwell 2003, {{ISBN|978-0-631-18117-0}} – cited by [http://necrometrics.com/pre1700a.htm#Manchu17c White] {{Webarchive|url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#Manchu17c |date=20 December 2017 }}.</ref>
{| class=wikitable
|-
! Deaths<br />(millions)
! Date
! War
|-
| {{right|70–85}} || 1939–1945 || [[World War II]] (see [[World War II casualties]])
|-
| {{right|60}} || 13th century || [[Mongol Empire|Mongol Conquests]] (see [[Mongol invasions and conquests|Mongol invasions]] and [[Tatar invasions]])<ref>Ping-ti Ho, "An Estimate of the Total Population of Sung-Chin China", in ''Études Song'', Series 1, No 1, (1970) pp. 33–53.</ref><ref>{{cite web |url=http://necrometrics.com/pre1700a.htm#Mongol |title=Mongol Conquests |publisher=Users.erols.com |access-date=2011-01-24 |archive-date=20 December 2017 |archive-url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#Mongol |url-status=live }}</ref><ref>{{cite news|url=http://www.globalwebpost.com/genocide1971/articles/general/worst_massacres.htm |archive-url=https://web.archive.org/web/20030517105614/http://www.globalwebpost.com/genocide1971/articles/general/worst_massacres.htm |url-status=dead |archive-date=17 May 2003 |title=The world's worst massacres Whole Earth Review |access-date=2011-01-24 |year=1987 }}</ref>
|-
| {{right|40}} || 1850–1864 || [[Taiping Rebellion]] (see [[Dungan Revolt (1862–1877)|Dungan Revolt]])<ref>{{cite encyclopedia |url=http://concise.britannica.com/ebc/article-9380148/Taiping |title=Taiping Rebellion – Britannica Concise |encyclopedia=Britannica |access-date=2011-01-24 |archive-date=15 December 2007 |archive-url=https://web.archive.org/web/20071215124111/http://concise.britannica.com/ebc/article-9380148/Taiping |url-status=dead }}</ref>
|-
| {{right|36}} || 755–763 || [[An Lushan Rebellion]] (death toll uncertain)<ref>{{cite web |url=http://necrometrics.com/pre1700a.htm#AnLushan |title=Selected Death Tolls for Wars, Massacres and Atrocities Before the 20th Century |publisher=Users.erols.com |access-date=2011-01-24 |archive-date=20 December 2017 |archive-url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#AnLushan |url-status=live }}</ref>
|-
| {{right|25}} || 1616–1662 || [[Qing dynasty]] conquest of [[Ming dynasty]]<ref name="users.erols" />
|-
| {{right|15–22}} || 1914–1918 ||[[World War I]] (see [[World War I casualties]])<ref name = Britannica>{{Cite web|url=https://www.britannica.com/event/World-War-I/Killed-wounded-and-missing|title=World War I - Killed, wounded, and missing &#124; Britannica|website=Britannica.com|access-date=5 December 2021}}</ref> 
|-
| {{right|20}} || 1937–1945 || [[Second Sino-Japanese War]]<ref>{{cite news |url=https://www.bbc.co.uk/history/worldwars/wwtwo/nuclear_01.shtml |title=Nuclear Power: The End of the War Against Japan |work=BBC News |access-date=2011-01-24 |archive-date=28 November 2015 |archive-url=https://web.archive.org/web/20151128194317/http://www.bbc.co.uk/history/worldwars/wwtwo/nuclear_01.shtml |url-status=live }}</ref>
|-
| {{right|20}} || 1370–1405 || Conquests of [[Tamerlane]]<ref>{{cite web |url=http://necrometrics.com/pre1700a.htm#Timur |title=Timur Lenk (1369–1405) |publisher=Users.erols.com |access-date=2011-01-24 |archive-date=20 December 2017 |archive-url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#Timur |url-status=live }}</ref><ref>[http://necrometrics.com/pre1700a.htm#Asian Matthew White's website] {{Webarchive|url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#Asian |date=20 December 2017 }} (a compilation of scholarly death toll estimates).</ref>
|-
| {{right|20.77}} || 1862–1877 || [[Dungan Revolt (1862–1877)|Dungan Revolt]]<ref>{{cite book |title=《中国人口史》 |language=zh |volume=5《清时期》 |page=635 |author=曹树基}} {{Full citation needed|date=February 2016}}</ref><ref name="Lu">{{cite news |title=同治光绪年间陕西人口的损失 |language=zh |author = 路伟东}}{{Full citation needed|date=February 2016}}</ref>
|-
| {{right|5–9}} || 1917–1922 || [[Russian Civil War]] and [[Allied intervention in the Russian Civil War|Foreign Intervention]]<ref>{{cite web|url=https://spartacus-educational.com/RUScivilwar.htm |title=Russian Civil War |publisher=Spartacus-Educational.com |access-date=2019-02-26 |url-status=live |archive-url=https://web.archive.org/web/20101205201225/http://www.spartacus.schoolnet.co.uk/RUScivilwar.htm |archive-date=5 December 2010 }}</ref>
|}

===On military personnel===
[[Military personnel]] subject to combat in war often suffer mental and physical injuries, including depression, [[posttraumatic stress disorder]], disease, injury, and death.

{{Quote|In every war in which American soldiers have fought in, the chances of becoming a psychiatric casualty – of being debilitated for some period of time as a consequence of the stresses of military life – were greater than the chances of being killed by enemy fire.|''No More Heroes'', Richard Gabriel<ref name="War"/>}}

Swank and Marchand's World War II study found that after sixty days of continuous combat, 98% of all surviving military personnel will become psychiatric casualties. Psychiatric casualties manifest themselves in fatigue cases, confusional states, conversion hysteria, anxiety, obsessional and compulsive states, and character disorders.<ref name="autogenerated1996">{{cite book|last=Lt. Col. Dave Grossman|title=On Killing – The Psychological Cost of Learning to Kill in War & Society|publisher= Little, Brown & Co. |year=1996}}</ref>

{{Quote|One-tenth of mobilised American men were hospitalised for mental disturbances between 1942 and 1945, and after thirty-five days of uninterrupted combat, 98% of them manifested psychiatric disturbances in varying degrees.|''14–18: Understanding the Great War'', Stéphane Audoin-Rouzeau, Annette Becker<ref name="War"/>}}

Additionally, it has been estimated anywhere from 18% to 54% of Vietnam war veterans suffered from [[posttraumatic stress disorder]].<ref name="autogenerated1996"/> 

Based on 1860 census figures, 8% of all white American males aged 13 to 43 died in the [[American Civil War]], including about 6% in the North and approximately 18% in the South.<ref>{{cite book|author=Maris Vinovskis|title=Toward a Social History of the American Civil War: Exploratory Essays|url=https://books.google.com/books?id=9D4TAwc93VoC|access-date=31 May 2012|date=1990|publisher=Cambridge University Press|isbn=978-0-521-39559-5|archive-date=26 May 2013|archive-url=https://web.archive.org/web/20130526101206/http://books.google.com/books?id=9D4TAwc93VoC|url-status=live}}</ref> The war remains the deadliest conflict in American history, resulting in the deaths of 620,000 military personnel. [[United States military casualties of war]] since 1775 have totaled over two million. Of the 60&nbsp;million European military personnel who were mobilized in [[World War I]], 8&nbsp;million were killed, 7&nbsp;million were permanently disabled, and 15&nbsp;million were seriously injured.<ref>Kitchen, Martin (2000), ''[http://www.jimmyatkinson.com/papers/versaillestreaty.html#endnotes The Treaty of Versailles and its Consequences] {{webarchive|url=https://web.archive.org/web/20080512224100/http://www.jimmyatkinson.com/papers/versaillestreaty.html |date=12 May 2008 }}'', New York: Longman.</ref>

[[File:DeadCrowIndians1874.jpg|thumb|left|The remains of dead [[Crow Indians]] killed and scalped by Sioux {{Circa|1874}}]]
During [[Napoleon I of France|Napoleon]]'s retreat from Moscow, more French military personnel died of [[typhus]] than were killed by the Russians.<ref>[https://web.archive.org/web/20091106154259/http://entomology.montana.edu/historybug/TYPHUS-Conlon.pdf The Historical Impact of Epidemic Typhus]. Joseph M. Conlon.</ref> Of the 450,000 soldiers who crossed the [[Neman River|Neman]] on 25 June 1812, less than 40,000 returned. More military personnel were killed from 1500 to 1914 by typhus than from military action.<ref name="TIME Magazine 1940">[https://web.archive.org/web/20090921004137/http://www.time.com/time/magazine/article/0,9171,794989,00.html War and Pestilence]. ''Time''.</ref> In addition, if it were not for modern medical advances there would be thousands more dead from disease and infection. For instance, during the [[Seven Years' War]], the [[Royal Navy]] reported it conscripted 184,899 sailors, of whom 133,708 (72%) died of disease or were 'missing'.<ref>A. S. Turberville (2006). ''Johnson's England: An Account of the Life & Manners of His Age''. p. 53. {{ISBN|1-4067-2726-1}}</ref> It is estimated that between 1985 and 1994, 378,000 people per year died due to war.<ref>{{cite journal |vauthors=Obermeyer Z, Murray CJ, Gakidou E |title=Fifty years of violent war deaths from Vietnam to Bosnia: analysis of data from the world health survey programme |journal=BMJ |volume=336 |issue=7659 |pages=1482–86 |date=June 2008 |pmid=18566045 |pmc=2440905 |doi=10.1136/bmj.a137 }}</ref>

===On civilians===
{{see also|Civilian casualties}}
[[File:The Hanging by Jacques Callot.jpg|right|thumb|upright=1.2|''[[Les Grandes Misères de la guerre]]'' depict the destruction unleashed on civilians during the [[Thirty Years' War]].]]
Most wars have resulted in significant loss of life, along with destruction of infrastructure and resources (which may lead to [[famine]], disease, and death in the [[civilian]] [[Population (human biology)|population]]). During the [[Thirty Years' War]] in Europe, the population of the [[Holy Roman Empire]] was reduced by 15 to 40 percent.<ref>[http://necrometrics.com/pre1700a.htm#30YrW The Thirty Years War (1618–48)] {{Webarchive|url=https://web.archive.org/web/20171220023849/http://necrometrics.com/pre1700a.htm#30YrW |date=20 December 2017 }}, Alan McFarlane, The Savage Wars of Peace: England, Japan and the Malthusian Trap (2003).</ref><ref>[http://www.britannica.com/EBchecked/topic/195896/history-of-Europe/58335/Demographics#ref=ref310375 History of Europe – Demographics] {{Webarchive|url=https://web.archive.org/web/20130723052625/http://www.britannica.com/EBchecked/topic/195896/history-of-Europe/58335/Demographics#ref=ref310375 |date=23 July 2013 }}. Encyclopædia Britannica.</ref> Civilians in war zones may also be subject to war atrocities such as [[genocide]], while survivors may suffer the psychological aftereffects of witnessing the destruction of war. War also results in lower quality of life and worse health outcomes. A medium-sized conflict with about 2,500 battle deaths reduces civilian life expectancy by one year and increases [[infant mortality]] by 10% and [[malnutrition]] by 3.3%. Additionally, about 1.8% of the population loses access to [[drinking water]].<ref>{{cite journal|doi=10.1146/annurev-polisci-050317-064057|doi-access=free|title=The Consequences of Contention: Understanding the Aftereffects of Political Conflict and Violence|year=2019|last1=Davenport|first1=Christian|author-link=Christian Davenport|last2=Mokleiv Nygård|first2=Håvard|first3=Hanne|last3=Fjelde|last4=Armstrong|first4=David|journal=Annual Review of Political Science|volume=22|pages=361–377}}</ref>

Most estimates of [[World War II casualties]] indicate around 60&nbsp;million people died, 40&nbsp;million of whom were civilians.<ref>{{cite web|title=World War II Fatalities|url=http://www.secondworldwar.co.uk/casualty.html|access-date=2007-04-20|archive-date=22 April 2007|archive-url=https://web.archive.org/web/20070422000628/http://www.secondworldwar.co.uk/casualty.html|url-status=dead}}</ref> Deaths in the [[Soviet Union]] were around [[World War II casualties of the Soviet Union|27{{nbsp}}million]].<ref>{{cite news|url=http://news.bbc.co.uk/2/hi/europe/4530565.stm|title=Leaders mourn Soviet wartime dead|date=9 May 2005|access-date=6 January 2010|work=BBC News|archive-date=22 December 2019|archive-url=https://web.archive.org/web/20191222043852/http://news.bbc.co.uk/2/hi/europe/4530565.stm|url-status=live}}</ref> Since a high proportion of those killed were young men who had not yet fathered any children, population growth in the postwar Soviet Union was much lower than it otherwise would have been.<ref>{{cite book|first=Geoffrey A.|last=Hosking|author-link=Geoffrey Hosking|title=Rulers And Victims: The Russians in the Soviet Union|url=https://books.google.com/books?id=CDMVMqDvp4QC&pg=PA242|access-date=31 May 2012|year=2006|publisher=Harvard University Press|isbn=978-0-674-02178-5|pages=242–|archive-date=5 September 2015|archive-url=https://web.archive.org/web/20150905171344/https://books.google.com/books?id=CDMVMqDvp4QC&pg=PA242|url-status=live}}</ref>

===Economic===
{{see also|Military Keynesianism}}
Once a war has ended, losing nations are sometimes required to pay [[war reparations]] to the victorious nations. In certain cases, land is ceded to the victorious nations. For example, the territory of [[Alsace-Lorraine]] has been traded between France and Germany on three different occasions.<ref>{{cite encyclopedia |url=https://www.britannica.com/place/Alsace-Lorraine |title=Alsace-Lorraine |encyclopedia=Encyclopædia Britannica |edition=Online |access-date=21 March 2022 |archive-date=20 March 2022 |archive-url=https://web.archive.org/web/20220320174427/https://www.britannica.com/place/Alsace-Lorraine |url-status=live }}</ref>

Typically, war becomes intertwined with the economy and many wars are partially or entirely based on economic reasons. The common view among economic historians is that the [[Great Depression]] ended with the advent of [[World War II]]. Many economists believe that government spending on the war caused or at least accelerated recovery from the Great Depression, though some consider that it did not play a very large role in the recovery, though it did help in reducing unemployment.<ref name="Britannica1">[https://www.britannica.com/EBchecked/topic/243118/Great-Depression "Great Depression"] {{Webarchive|url=https://web.archive.org/web/20150509121741/https://www.britannica.com/EBchecked/topic/243118/Great-Depression|date=9 May 2015}}, ''Encyclopædia Britannica.''</ref><ref name="Galbraith">Referring to the effect of World War II spending on the economy, economist [[John Kenneth Galbraith]] said, "One could not have had a better demonstration of the Keynesian ideas." {{cite video |url=https://www.pbs.org/wgbh/commandingheights/lo/story/ch_menu.html |title=Commanding Heights, see chapter 6 video or transcript |date=2002 |medium=TV documentary |publisher=[[Public Broadcasting Service|PBS]] |location=U.S. |people=[[Daniel Yergin]], William Cran (writers / producer)}}</ref><ref>{{Cite journal |last=Romer |first=Christina D. |author-link=Christina Romer |year=1992 |title=What Ended the Great Depression? |journal=Journal of Economic History |volume=52 |issue=4 |pages=757–784 |doi=10.1017/S002205070001189X |quote=fiscal policy was of little consequence even as late as 1942, suggests an interesting twist on the usual view that World War II caused, or at least accelerated, the recovery from the Great Depression.}}</ref><ref>{{Cite journal |last=Higgs |first=Robert |author-link=Robert Higgs |date=1 March 1992 |title=Wartime Prosperity? A Reassessment of the U.S. Economy in the 1940s |journal=The Journal of Economic History |volume=52 |issue=1 |pages=41–60 |doi=10.1017/S0022050700010251 |issn=1471-6372 |s2cid=154484756}}</ref> In most cases, such as the wars of Louis XIV, the [[Franco-Prussian War]], and [[World War I]], warfare primarily results in damage to the economy of the countries involved. For example, Russia's involvement in World War I took such a toll on the Russian economy that it almost collapsed and greatly contributed to the start of the [[Russian Revolution of 1917]].<ref>{{cite book|last1=Gatrell|first1=Peter|author-link1=:de:Peter Gatrell|title=Russia's First World War : A Social and Economic History|date=2014|publisher=Routledge|location=Hoboken, New Jersey |isbn=978-1317881391|page=270}}</ref>
====World War II====
[[File:Cyprian War.jpg|thumb|upright=.75|Ruins of [[Warsaw]]'s Napoleon Square in the aftermath of [[World War II]]]]
[[World War II]] was the most financially costly conflict in history; its belligerents cumulatively spent about a trillion U.S. dollars on the [[war effort]] (as adjusted to 1940 prices).<ref>{{cite web |last=Mayer |first=E. |date=2000 |url= http://emayzine.com/lectures/WWII.html |title=World War II course lecture notes|website=Emayzine.com |location=Victorville, California |publisher=Victor Valley College |archive-url=https://web.archive.org/web/20090301155526/http://emayzine.com/lectures/WWII.html |archive-date=1 March 2009 |access-date=4 July 2014}}</ref><ref>Coleman, P. (1999) [https://web.archive.org/web/20080302153205/http://members.aol.com/forcountry/ww2/wc1.htm "Cost of the War"], ''World War II Resource Guide'' (Gardena, California: The American War Library).</ref>
The [[Great Depression]] of the 1930s ended as nations increased their production of war materials.<ref>{{cite web |url=http://memory.loc.gov/learn/features/timeline/depwwii/depwar.html |title= Great Depression and World War II, 1929–1945 |publisher=Library of Congress |archive-url=https://web.archive.org/web/20071012023043/http://memory.loc.gov/learn/features/timeline/depwwii/depwar.html |archive-date=12 October 2007 |access-date=4 July 2014}}</ref>

By the end of the war, 70% of European industrial infrastructure was destroyed.<ref>{{cite book|author-link1=Marc Pilisuk|author1=Marc Pilisuk|author2=Jennifer Achord Rountree|title=Who Benefits from Global Violence and War: Uncovering a Destructive System|url=https://books.google.com/books?id=r9kNZrmG0E8C&pg=PA136|access-date=31 May 2012|year=2008|publisher=Greenwood Publishing Group|isbn=978-0-275-99435-8|pages=136–|archive-date=26 May 2013|archive-url=https://web.archive.org/web/20130526083711/http://books.google.com/books?id=r9kNZrmG0E8C&pg=PA136|url-status=live}}</ref> Property damage in the Soviet Union inflicted by the [[Eastern Front (World War II)|Axis invasion]] was estimated at a value of 679&nbsp;billion rubles. The combined damage consisted of complete or partial destruction of 1,710 cities and towns, 70,000 villages/hamlets, 2,508 church buildings, 31,850 industrial establishments, {{convert|40000|mi|0|abbr=on}} of railroad, 4100 railroad stations, 40,000 hospitals, 84,000 schools, and 43,000 public libraries.<ref>''[[The New York Times]]'', 9 February 1946, Volume 95, Number 32158.</ref>

==Theories of motivation==
{{see also|International relations theory}}

There are many theories about the motivations for war, but no consensus about which are most common.<ref>{{Cite journal|last=Levy |first=Jack S. |author-link=:de:Jack Levy |year=1989 |title=The Causes of War: A Review of Theories and Evidence |journal=Behavior, Society and Nuclear War |volume=I |page=295 |editor1-first=Philip E. |editor1-last=Tetlock |editor2-first=Jo L. |editor2-last=Husbands |editor3-first=Robert |editor3-last=Jervis |editor4-first=Paul C. |editor4-last=Stern |editor5-first=Charles |editor5-last=Tilly |access-date=4 May 2012 |url=http://fas-polisci.rutgers.edu/levy/1989%20Causes%20of%20War.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130922150721/http://fas-polisci.rutgers.edu/levy/1989%20Causes%20of%20War.pdf |archive-date=22 September 2013 }}</ref> Military theorist [[Carl von Clausewitz]] said, "Every age has its own kind of war, its own limiting conditions, and its own peculiar preconceptions."<ref>Clausewitz, Carl Von (1976), ''On War'' (Princeton University Press) p. 593.</ref>

===Psychoanalytic===
Dutch [[psychoanalyst]] [[Joost Meerloo]] held that, "War is often...a mass discharge of accumulated internal rage (where)...the inner fears of mankind are discharged in mass destruction."<ref name="meerloo">
|. A. M. Meerloo, M.D. ''The Rape of the Mind'' (2009) p. 134, Progressive Press, {{ISBN|978-1-61577-376-3}}.
</ref> Other psychoanalysts such as E.F.M. Durban and [[John Bowlby]] have argued human beings are inherently violent.<ref>Durbin, E.F.L. and John Bowlby. ''Personal Aggressiveness and War'' 1939.</ref> This aggressiveness is fueled by [[displacement (psychology)|displacement]] and [[Psychological projection|projection]] where a person transfers his or her grievances into bias and hatred against other [[race (human classification)|races]], [[religion]]s, [[nation]]s or [[Ideology|ideologies]]. By this theory, the nation state preserves order in the local society while creating an outlet for aggression through warfare.

The Italian psychoanalyst [[Franco Fornari]], a follower of [[Melanie Klein]], thought war was the paranoid or projective "elaboration" of mourning.<ref>(Fornari 1975).</ref> Fornari thought war and violence develop out of our "love need": our wish to preserve and defend the sacred object to which we are attached, namely our early mother and our fusion with her. For the adult, nations are the sacred objects that generate warfare. Fornari focused upon sacrifice as the essence of war: the astonishing willingness of human beings to die for their country, to give over their bodies to their nation.

Despite Fornari's theory that man's altruistic desire for self-sacrifice for a noble cause is a contributing factor towards war, few wars have originated from a desire for war among the general populace.<ref>Blanning, T.C.W. "The Origin of Great Wars." ''The Origins of the French Revolutionary Wars.'' p. 5.</ref> Far more often the general population has been reluctantly drawn into war by its rulers. One psychological theory that looks at the leaders is advanced by Maurice Walsh.<ref>Walsh, Maurice N. ''War and the Human Race.'' 1971.</ref> He argues the general populace is more neutral towards war and wars occur when leaders with a psychologically abnormal disregard for human life are placed into power. War is caused by leaders who seek war such as [[Napoleon I of France|Napoleon]] and [[Adolf Hitler|Hitler]]. Such leaders most often come to power in times of crisis when the populace opts for a decisive leader, who then leads the nation to war.

{{Quotation|Naturally, the common people don't want war; neither in Russia nor in England nor in America, nor for that matter in Germany. That is understood. But, after all, it is the leaders of the country who determine the policy and it is always a simple matter to drag the people along, whether it is a democracy or a fascist dictatorship or a Parliament or a Communist dictatorship. ... the people can always be brought to the bidding of the leaders. That is easy. All you have to do is tell them they are being attacked and denounce the pacifists for lack of patriotism and exposing the country to danger. It works the same way in any country.|
[[Hermann Göring]] at the [[Nuremberg trials]], 18 April 1946<ref>{{cite web|url=http://www.snopes.com/quotes/goering.htm|title=In an interview with Gilbert in Göring's jail cell during the Nuremberg War Crimes Trials (18 April 1946)|access-date=5 August 2015|date=2017-04-18}}</ref>}}

===Evolutionary===
{{See also|Prehistoric warfare}}

Several theories concern the evolutionary origins of warfare. There are two main schools: One sees organized warfare as emerging in or after the Mesolithic as a result of complex social organization and greater population density and [[competition]] over [[resource]]s; the other sees human warfare as a more ancient practice derived from common animal tendencies, such as territoriality and sexual competition.<ref>Peter Meyer. Social Evolution in Franz M. Wuketits and Christoph Antweiler (eds.) Handbook of Evolution. The Evolution of Human Societies and Cultures Wiley-VCH Verlag.</ref>

The latter school argues that since warlike behavior patterns are found in many primate species such as [[Common chimpanzee|chimpanzee]]s,<ref name="ApesWar">{{cite news| url=https://www.telegraph.co.uk/news/science/science-news/3317461/Apes-of-war...-is-it-in-our-genes.html| title=Apes of war...is it in our genes?| access-date=2010-02-06| location=London| work=The Daily Telegraph| first=Sanjida| last=O'Connell| date=7 January 2004| archive-date=4 September 2018| archive-url=https://web.archive.org/web/20180904192203/https://www.telegraph.co.uk/news/science/science-news/3317461/Apes-of-war...-is-it-in-our-genes.html| url-status=live}} Analysis of chimpanzee war behavior.</ref> as well as in many [[ant]] species,<ref name="AntsLandmines">{{citation | year = 1996| ssrn = 935783| title= Warrior Ants: The Enduring Threat of the Small War and the Land-mine| last1 = Anderson| first1 = Kenneth}} Scholarly comparisons between human and ant wars.</ref> group conflict may be a general feature of animal social behavior. Some proponents of the idea argue that war, while innate, has been intensified greatly by developments of technology and social organization such as weaponry and states.<ref>Johan M.G. van der Dennen. 1995. ''The Origin of War: Evolution of a Male-Coalitional Reproductive Strategy''. Origin Press, Groningen, 1995 chapters 1 & 2.</ref> A study even found that civil war did occur in wild chimpanzee colonies, leading to a violent split in a group of chimpanzees.<ref>{{Cite journal |last1=Sandel |first1=Aaron A. |last2=He |first2=Yixuan |last3=Ren |first3=Junpeng |last4=Kei |first4=Yik Lun |last5=Lee |first5=Kevin C. |last6=Clark |first6=Isabelle R. |last7=Reddy |first7=Rachna B. |last8=Negrey |first8=Jacob D. |last9=Birungi |first9=Charles |last10=Apamaku |first10=Blessing A. |last11=Kanweri |first11=Diana |last12=Kalunga |first12=Davis |last13=Aliganyira |first13=Christopher |last14=Ramírez-Amaya |first14=Sebastián |last15=Nakayima |first15=Phionah |date=2026-04-09 |title=Lethal conflict after group fission in wild chimpanzees |url=https://www.science.org/doi/10.1126/science.adz4944 |journal=Science |volume=392 |issue=6794 |pages=216–220 |doi=10.1126/science.adz4944|pmid=41955363 |bibcode=2026Sci...392..216S |url-access=subscription }}</ref>

Psychologist and linguist [[Steven Pinker]] argued that war-related behaviors may have been naturally selected in the ancestral environment due to the benefits of victory.{{refn|group=lower-alpha|name = fn1|
The argument is made from pages 314 to 332 of [[The Blank Slate]].<ref>{{cite book |last=Pinker  |first=Steven |author-link=Steven Pinker |date=2002 |title=The Blank Slate: The Modern Denial of Human Nature |url= |location=London |publisher=The Penguin Group |pages=314–332 |isbn=0-713-99256-5}}</ref> Relevant quotes include on p332 "The first step in understanding violence is to set aside our abhorrence of it long enough to examine why it can sometimes pay off in evolutionary terms.", "Natural selection is powered by competition, which means that the products of natural selection{{snd}}survival machines, in Richard Dawkins metaphor{{snd}}should, by default, do whatever helps them survive and reproduce.". On p323 "If an obstacle stands in the way of something an organism needs, it should neutralize the obstacle by disabling or eliminating it.", "Another human obstacle consists of men monopolozing women who could otherwise be taken as wives.", "The competition can be violent". On p324 "So people have invented, and perhaps evolved, an alternate defense: the advertised deterrence policy known as ''lex talionis'', the law of retaliation, familiar from the biblical injunction "An eye for an eye, a tooth for a tooth." If you can credibly say to potential adversaries, "We won't attack first, but if we are attacked, we will survive and strike back," you remove Hobbes's first two incentives for quarrel, gain and mistrust.". On p326 "Also necessary for vengeance to work as a deterrent is that the willingness to pursue it be made public, because the whole point of deterrence is to give would-be attackers second thoughts ''beforehand''. And this brings us to Hobbes's final reason for quarrel. Thirdly, glory{{snd}}though a more accurate word would be "honor"."}} He also argued that in order to have credible [[deterrence theory|deterrence]] against other groups (as well as on an individual level), it was important to have a reputation for retaliation, causing humans to develop instincts for [[revenge]] as well as for protecting a group's (or an individual's) reputation ("[[honor]]").{{refn|name = fn1|group=lower-alpha}}

[[File:Reproduction of Bonampak murals (panorama).JPG|thumb|left|upright=1.2|Increasing population and constant warfare among the [[Maya warfare|Maya]] city-states over resources may have contributed to the eventual [[Classic Maya collapse|collapse]] of the [[Maya civilization]] by 900 CE.]]
Crofoot and Wrangham have argued that warfare, if defined as group interactions in which "coalitions attempt to aggressively dominate or kill members of other groups", is a characteristic of most human societies. Those in which it has been lacking "tend to be societies that were politically dominated by their neighbors".<ref name="HumanPrimateAggression">''Mind the Gap: Tracing the Origins of Human Universals'' By Peter M. Kappeler, Joan B. Silk, 2009, Chapter 8, "Intergroup Aggression in Primates and Humans; The Case for a Unified Theory", Margaret C. Crofoot and Richard W. Wrangham.</ref>

[[Ashley Montagu]] strongly denied universalistic instinctual arguments, arguing that social factors and childhood socialization are important in determining the nature and presence of warfare. Thus, he argues, warfare is not a universal human occurrence and appears to have been a historical invention, associated with certain types of human societies.<ref>Montagu, Ashley (1976), ''The Nature of Human Aggression'' (Oxford University Press).</ref> Montagu's argument is supported by ethnographic research conducted in societies where the concept of aggression seems to be entirely absent, e.g. the [[Chewong]] and [[Semai people|Semai]] of the Malay peninsula.<ref>Howell, Signe and Roy Willis, eds. (1989) ''Societies at Peace: Anthropological Perspectives''. London: Routledge.</ref> Bobbi S. Low has observed correlation between warfare and education, noting societies where warfare is commonplace encourage their children to be more aggressive.<ref>[http://sitemaker.umich.edu/eugene.burnstein/files/35._low_93_evolutionaryperspective_war.pdf "An Evolutionary Perspective on War"] {{Webarchive|url=https://web.archive.org/web/20151016052509/http://sitemaker.umich.edu/eugene.burnstein/files/35._low_93_evolutionaryperspective_war.pdf |date=16 October 2015 }}, Bobbi S. Low, published in ''Behavior, Culture, and Conflict in World Politics'', The University of Michigan Press, p. 22</ref>

===Economic===
[[File:Disabled Iraqi T-54A, T-55, Type 59 or Type 69 tank and burning Kuwaiti oil field.jpg|thumb|Kuwaiti [[oil well]]s on fire during the [[Gulf War]], 1 March 1991]]
{{See Also|Resource war}}
War can be seen as a growth of economic competition in a competitive international system. In this view wars begin as a pursuit of markets for [[natural resource]]s and for wealth. War has also been linked to [[economic development]] by economic historians and development economists studying [[state-building]] and [[fiscal capacity]].<ref>{{cite journal |last1=Johnson |first1=Noel D. |last2=Koyama |first2=Mark  |date=April 2017 |title=States and economic growth: Capacity and constraints |url= |journal= Explorations in Economic History|volume= 64|issue= |pages=1–20 |doi=10.1016/j.eeh.2016.11.002 |access-date=}}</ref> While this theory has been applied to many conflicts, such counter arguments become less valid as the increasing mobility of capital and information level the distributions of wealth worldwide, or when considering that it is relative, not absolute, wealth differences that may fuel wars. There are those on the extreme [[right (politics)|right]] of the political spectrum who provide support, fascists in particular, by asserting a natural right of a strong nation to whatever the weak cannot hold by force.<ref>[[Roger Griffin]] and Matthew Feldman, eds., ''Fascism: Fascism and Culture'', New York: [[Routledge]], 2004.</ref><ref>Hawkins, Mike. ''[[Social Darwinism]] in European and American Thought, 1860–1945: Nature as Model and Nature as Threat'', [[Cambridge University Press]], 1997.</ref>
===Marxist===
{{Main|Marxist explanations of warfare}}

The [[Marxism|Marxist]] theory of war is quasi-economic in that it states all modern wars are caused by competition for resources and markets between great (imperialist) powers, claiming these wars are a natural result of [[capitalism]]. Marxist economists [[Karl Kautsky]], [[Rosa Luxemburg]], [[Rudolf Hilferding]] and [[Vladimir Lenin]] theorized that [[imperialism]] was the result of capitalist countries needing new [[Market (economics)|markets]]. Expansion of the [[means of production]] is only possible if there is a corresponding growth in [[consumer demand]]. Since the workers in a [[capitalist economy]] would be unable to fill the demand, producers must expand into non-capitalist markets to find consumers for their goods, hence driving imperialism.<ref>{{cite web |url=http://www.marxists.org/archive/mandel/1955/08/imp-crit.html |title=The Marxist Theory of Imperialism and its Critics |first=Einde |last=O'Callaghan |publisher=[[Marxists Internet Archive]] |date=25 October 2007 |access-date=24 April 2011 |archive-date=8 July 2017 |archive-url=https://web.archive.org/web/20170708100958/https://www.marxists.org/archive/mandel/1955/08/imp-crit.html |url-status=live }}</ref>

===Demographic===
<!-- This section is linked from [[The Clash of Civilizations|Clash of Civilizations]] -->

Demographic theories can be grouped into two classes, Malthusian and youth bulge theories:

====Malthusian====

[[Malthusian catastrophe|Malthusian theories]] see expanding population and scarce resources as a source of violent conflict. [[Pope Urban II]] in 1095, on the eve of the [[First Crusade]], advocating Crusade as a solution to European overpopulation, said:
{{quote|For this land which you now inhabit, shut in on all sides by the sea and the mountain peaks, is too narrow for your large population; it scarcely furnishes food enough for its cultivators. Hence it is that you murder and devour one another, that you wage wars, and that many among you perish in civil strife. Let hatred, therefore, depart from among you; let your quarrels end. Enter upon the road to the Holy Sepulchre; wrest that land from a wicked race, and subject it to yourselves.<ref>{{cite book |title=Lend me your ears: great speeches in history |first=William |last=Safire |publisher=[[W. W. Norton & Company]] |year=2004 |isbn=978-0-393-05931-1 |page=94}}</ref>}}

This is one of the earliest expressions of what has come to be called the Malthusian theory of war, in which wars are caused by expanding populations and limited resources. [[Thomas Malthus]] (1766–1834) wrote that populations always increase until they are limited by war, disease, or [[famine]].<ref>{{cite book |title=Geography: an integrated approach |first=David |last=Waugh |publisher=[[Nelson Thornes]] |year=2000 |isbn=978-0-17-444706-1 |page=378}}</ref> The violent [[herder–farmer conflicts in Nigeria]], [[Mali]], [[Sudanese nomadic conflicts|Sudan]] and other countries in the [[Sahel]] region have been exacerbated by [[land degradation]] and population growth.<ref>{{cite news |title=In Mali, waning fortunes of Fulani herders play into Islamist hands |url=https://www.reuters.com/article/us-mali-fulani/in-mali-waning-fortunes-of-fulani-herders-play-into-islamist-hands-idUSKBN13F0L2 |work=Reuters |date=20 November 2016 |access-date=31 March 2019 |archive-date=30 March 2019 |archive-url=https://web.archive.org/web/20190330131908/https://www.reuters.com/article/us-mali-fulani/in-mali-waning-fortunes-of-fulani-herders-play-into-islamist-hands-idUSKBN13F0L2 |url-status=live }}</ref><ref>{{cite news |title=How Climate Change Is Spurring Land Conflict in Nigeria |url=https://time.com/5324712/climate-change-nigeria/ |magazine=Time |date=28 June 2018 |access-date=31 March 2019 |archive-date=4 March 2021 |archive-url=https://web.archive.org/web/20210304231237/https://time.com/5324712/climate-change-nigeria/ |url-status=live }}</ref><ref>{{cite news |title=The Deadliest Conflict You've Never Heard of |url=https://www.foreignaffairs.com/articles/nigeria/2019-01-23/deadliest-conflict-youve-never-heard |work=[[Foreign Policy]] |date=23 January 2019 |access-date=31 March 2019 |archive-date=18 February 2019 |archive-url=https://web.archive.org/web/20190218125507/https://www.foreignaffairs.com/articles/nigeria/2019-01-23/deadliest-conflict-youve-never-heard |url-status=live }}</ref>

====Youth bulge====
[[File:2017 world map, median age by country.svg|thumb|upright=1.4|[[Population pyramid|Median age]] by country. War reduces life expectancy. A youth bulge is evident for [[Demographics of Africa|Africa]], and to a lesser extent in some countries in West Asia, South Asia, Southeast Asia and Central America.]]

According to [[Gunnar Heinsohn|Heinsohn]], who proposed [[youth bulge]] theory in its most generalized form, a youth bulge occurs when 30 to 40 percent of the males of a nation belong to the "fighting age" cohorts from 15 to 29 years of age. It will follow periods with [[total fertility rate]]s as high as 4–8 children per woman with a 15–29-year delay.<ref>Helgerson, John L. (2002): "The National Security Implications of Global Demographic Trends" [http://www.au.af.mil/au/awc/awcgate/cia/helgerson2.htm] {{Webarchive|url=https://web.archive.org/web/20171010064934/http://www.au.af.mil/au/awc/awcgate/cia/helgerson2.htm|date=10 October 2017}}.</ref><ref>Heinsohn, G. (2006): "Demography and War" [https://de.scribd.com/doc/310263543/Gunnar-Heinsohn-Demography-and-War (online)] {{Webarchive|url=https://web.archive.org/web/20160512185458/https://de.scribd.com/doc/310263543/Gunnar-Heinsohn-Demography-and-War |date=12 May 2016 }}.</ref> Heinsohn saw both past "Christianist" European colonialism and imperialism, as well as today's Islamist civil unrest and terrorism as results of high birth rates producing youth bulges.<ref>Heinsohn, G. (2005): "Population, Conquest and Terror in the 21st Century" [https://de.scribd.com/doc/310265022/Gunnar-Heinsohn-Population-Conquest-and-Terror-in-the-21st-Century (online)] {{Webarchive|url=https://web.archive.org/web/20160513082119/https://de.scribd.com/doc/310265022/Gunnar-Heinsohn-Population-Conquest-and-Terror-in-the-21st-Century |date=13 May 2016 }}.</ref> 

Among prominent historical events that have been attributed to youth bulges are the role played by the historically large youth cohorts in the rebellion and revolution waves of early modern Europe, including the [[French Revolution]] of 1789,<ref>{{cite book|author=Jack A. Goldstone|title=Revolution and Rebellion in the Early Modern World|url=https://books.google.com/books?id=M-T9dR7nWDUC|access-date=31 May 2012|date= 1993|publisher=University of California Press|isbn=978-0-520-08267-0|archive-date=26 May 2013|archive-url=https://web.archive.org/web/20130526133145/http://books.google.com/books?id=M-T9dR7nWDUC|url-status=live}}</ref> and the effect of economic depression upon the largest German youth cohorts ever in explaining the rise of [[Nazism]] in Germany in the 1930s.<ref>Moller, Herbert (1968): 'Youth as a Force in the Modern World', ''Comparative Studies in Society and History'' 10: 238–60; 240–44.</ref> The 1994 [[Rwandan genocide]] has also been analyzed as following a massive youth bulge.<ref>Diessenbacher, Hartmut (1994): ''Kriege der Zukunft: Die Bevölkerungsexplosion gefährdet den Frieden''. Muenchen: Hanser 1998; see also (criticizing youth bulge theory) Marc Sommers (2006): "Fearing Africa's Young Men: The Case of Rwanda." The World Bank: Social Development Papers – Conflict Prevention and Reconstruction, Paper No. 32, January 2006 [http://www.eldis.org/vfile/upload/1/document/0708/DOC21389.pdf] {{Webarchive|url=https://web.archive.org/web/20171010064638/http://www.eldis.org/vfile/upload/1/document/0708/DOC21389.pdf|date=10 October 2017}}.</ref> Youth bulge theory has been subjected to statistical analysis by the World Bank,<ref>[[Henrik Urdal|Urdal, Henrik]] (2004): "The Devil in the Demographics: The Effect of Youth Bulges on Domestic Armed Conflict", [https://web.archive.org/web/20041118212538/http://www-wds.worldbank.org/servlet/WDSContentServer/WDSP/IB/2004/07/28/000012009_20040728162225/Rendered/PDF/29740.pdf].</ref> [[Population Action International]],<ref>Population Action International: "The Security Demographic: Population and Civil Conflict after the Cold War"[http://pai.org/wp-content/uploads/2012/01/The_Security_Demographic_Population_and_Civil_Conflict_After_the_Cold_War-1.pdf] {{Webarchive|url=https://web.archive.org/web/20171010064933/http://pai.org/wp-content/uploads/2012/01/The_Security_Demographic_Population_and_Civil_Conflict_After_the_Cold_War-1.pdf|date=10 October 2017}}.</ref> and the [[Berlin Institute for Population and Development]].<ref>Kröhnert, Steffen (2004): "Warum entstehen Kriege? Welchen Einfluss haben demografische Veränderungen auf die Entstehung von Konflikten?" [http://www.berlin-institut.org/fileadmin/user_upload/Jugend_und_Kriegsgefahr/Warum_entstehen_Kriege.pdf] {{Webarchive|url=https://web.archive.org/web/20180904230108/https://www.berlin-institut.org/fileadmin/user_upload/Jugend_und_Kriegsgefahr/Warum_entstehen_Kriege.pdf|date=4 September 2018}}.</ref> Youth bulge theories have been criticized as leading to racial, gender and age discrimination.<ref>Hendrixson, Anne: "Angry Young Men, Veiled Young Women: Constructing a New Population Threat" [http://www.thecornerhouse.org.uk/item.shtml?x=85999] {{Webarchive|url=https://web.archive.org/web/20100530200503/http://www.thecornerhouse.org.uk/item.shtml?x=85999|date=30 May 2010}}.</ref>
===Cultural===
[[Geoffrey Parker (historian)|Geoffrey Parker]] argues that what distinguishes the "Western way of war" based in Western Europe chiefly allows historians to explain its extraordinary success in conquering most of the world after 1500:<blockquote> The Western way of war rests upon five principal foundations: technology, discipline, a highly aggressive military tradition, a remarkable capacity to innovate and to respond rapidly to the innovation of others and{{snd}}from about 1500 onward{{snd}}a unique system of war finance. The combination of all five provided a formula for military success....The outcome of wars has been determined less by technology, then by better war plans, the achievement of surprise, greater economic strength, and above all superior discipline.<ref>Geoffrey Parker, "Introduction" in Parker, ed. ''The Cambridge illustrated history of warfare'' (Cambridge University Press 1995) pp. 2–11, [https://archive.org/details/cambridgeillustr0000unse_f0h1 online]. </ref></blockquote>

Parker argues that Western armies were stronger because they emphasized discipline, that is, "the ability of a formation to stand fast in the face of the enemy, where they're attacking or being attacked, without giving way to the natural impulse of fear and panic." Discipline came from drills and marching in formation, target practice, and creating small "artificial kinship groups: such as the company and the platoon, to enhance psychological cohesion and combat efficiency.<ref>Parker, :Introduction: pp. 2, 3.</ref>

===Rationalist===
{{see also|Bargaining model of war}}
[[Rationalism (international relations)|Rationalism]] is an [[international relations theory]] or framework. Rationalism (and [[Neorealism (international relations)]]) operate under the assumption that states or international actors are rational, seek the best possible outcomes for themselves, and desire to avoid the costs of war.<ref name=":0">{{Cite journal|title = Rationalist Explanations for War|last = Fearon|first = James D.|date = Summer 1995|journal = International Organization |volume=49 |issue=3 |pages=379–414|doi = 10.1017/s0020818300033324|jstor=2706903| s2cid=38573183 }}</ref> Under one [[game theory]] approach, rationalist theories posit all actors can [[Bargaining model of war|bargain]], would be better off if war did not occur, and likewise seek to understand why war nonetheless reoccurs. Under another rationalist game theory without bargaining, the [[peace war game]], optimal strategies can still be found that depend upon number of iterations played. In "Rationalist Explanations for War", [[James Fearon]] examined three rationalist explanations for why some countries engage in war:
* Issue indivisibilities
* Incentives to misrepresent or [[information asymmetry]]
* Commitment problems<ref name=":0" />

"Issue indivisibility" occurs when the two parties cannot avoid war by bargaining, because the thing over which they are fighting cannot be shared between them, but only owned entirely by one side or the other. "[[Information asymmetry]] with incentives to misrepresent" occurs when two countries have secrets about their individual capabilities, and do not agree on either: who would win a war between them, or the magnitude of state's victory or loss. For instance, [[Geoffrey Blainey]] argues that war is a result of miscalculation of strength. He cites historical examples of war and demonstrates, "war is usually the outcome of a diplomatic crisis which cannot be solved because both sides have conflicting estimates of their bargaining power."<ref>{{cite book|author=Geoffrey Blainey|title=Causes of War|url=https://books.google.com/books?id=Mcknp3tt0LMC&pg=PA114|year=1988|edition=3rd|page=114|publisher=Simon and Schuster |access-date=2016-03-19|isbn=978-0029035917|archive-date=1 January 2016|archive-url=https://web.archive.org/web/20160101210934/https://books.google.com/books?id=Mcknp3tt0LMC&pg=PA114|url-status=live}}</ref> Thirdly, bargaining may fail due to the states' inability to make credible commitments.<ref>{{cite journal | last1 = Powell | first1 = Robert | year = 2002 | title = Bargaining Theory and International Conflict | journal = Annual Review of Political Science | volume = 5 | pages = 1–30 | doi = 10.1146/annurev.polisci.5.092601.141138 |doi-access=free}}</ref>

Within the rationalist tradition, some theorists have suggested that individuals engaged in war suffer a normal level of [[cognitive bias]],<ref>Chris Cramer, 'Civil War is Not a Stupid Thing', {{ISBN|978-1850658214}}.</ref> but are still "as rational as you and me".<ref>From point 10 of [http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ Modern Conflict is Not What You Think (article)] {{Webarchive|url=https://web.archive.org/web/20160222013124/http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ |date=22 February 2016 }}, accessed 16 December 2014.</ref> According to philosopher [[Iain King]], "Most instigators of conflict overrate their chances of success, while most participants underrate their chances of injury...."<ref>Quote from [[Iain King]], in [http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ Modern Conflict is Not What You Think] {{Webarchive|url=https://web.archive.org/web/20160222013124/http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ |date=22 February 2016 }}.</ref> King asserts that "Most catastrophic military decisions are rooted in [[groupthink]]" which is faulty, but still rational.<ref>Point 6 in [http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ Modern Conflict is Not What You Think] {{Webarchive|url=https://web.archive.org/web/20160222013124/http://asiafoundation.org/in-asia/2014/12/10/modern-conflict-is-not-what-you-think/ |date=22 February 2016 }}.</ref> The rationalist theory focused around bargaining, which is currently under debate. The Iraq War proved to be an anomaly that undercuts the validity of applying rationalist theory to some wars.<ref>{{cite journal|last1=Lake|first1=David A.|title=Two Cheers for Bargaining Theory: Assessing Rationalist Explanations of the Iraq War|journal=International Security|date=November 2010|pages=7–52|doi=10.1162/isec_a_00029|volume=35|issue=3|s2cid=1096131}}</ref>

===Political science===
The statistical analysis of war was pioneered by [[Lewis Fry Richardson]] following [[World War I]]. More recent databases of wars and armed conflict have been assembled by the [[Correlates of War]] Project, Peter Brecke and the [[Uppsala Conflict Data Program]].<ref name="UCDP">{{Cite web|url=https://www.pcr.uu.se/research/ucdp/about-ucdp/|title=Uppsala Conflict Data Program – About|access-date=2019-04-09|archive-url=https://web.archive.org/web/20190403211437/http://pcr.uu.se/research/ucdp/about-ucdp|archive-date=2019-04-03|url-status=live}}</ref> The following subsections consider causes of war from system, societal, and individual levels of analysis. This kind of division was first proposed by [[Kenneth Waltz]] in ''[[Man, the State, and War]]'' and has been often used by political scientists since then.<ref name=Levy1998 />{{rp|143}}

====System-level====

There are several different [[international relations theory]] schools. Supporters of [[realism in international relations]] argue that the motivation of states is the quest for security, and conflicts can arise from the inability to distinguish defense from offense, which is called the [[security dilemma]].<ref name=Levy1998>{{cite journal|last1=Levy|first1=Jack S.|author-link1=:de:Jack Levy|title=The Causes of War and the Conditions of Peace|journal=Annual Review of Political Science|date=Jun 1998|volume=1|pages=139–65|doi=10.1146/annurev.polisci.1.1.139|doi-access=free}}</ref>{{rp|145}}

Within the realist school as represented by scholars such as [[Henry Kissinger]] and [[Hans Morgenthau]], and the [[neorealism (international relations)|neorealist]] school represented by scholars such as [[Kenneth Waltz]] and [[John Mearsheimer]], two main sub-theories are:
# [[Balance of power (international relations)|Balance of power]] theory: States have the goal of preventing a single state from becoming a hegemon, and war is the result of the would-be hegemon's persistent attempts at power acquisition. In this view, an international system with more equal distribution of power is more stable, and "movements toward unipolarity are destabilizing."<ref name=Levy1998 />{{rp|147}} However, evidence has shown power [[Polarity (international relations)|polarity]] is not actually a major factor in the occurrence of wars.<ref name=Levy1998 />{{rp|147–48}}
# [[Power transition theory]]: Hegemons impose stabilizing conditions on the world order, but they eventually decline, and war occurs when a declining hegemon is challenged by another rising power or aims to pre-emptively suppress them. On this view, unlike for balance-of-power theory, wars become ''more'' probable when power is more equally distributed. This "power preponderance" hypothesis has empirical support.<ref name=Levy1998 />{{rp|148}}
The two theories are not mutually exclusive and may be used to explain disparate events according to the circumstance.<ref name=Levy1998 />{{rp|148}} [[Liberalism (international relations)|Liberalism]] as it relates to international relations emphasizes factors such as [[International trade|trade]], and its role in disincentivizing conflict which will damage economic relations. Critics respond that military force may sometimes be at least as effective as trade at achieving economic benefits, especially historically if not as much today.<ref name=Levy1998 />{{rp|149}} Furthermore, trade relations which result in a high level of dependency may escalate tensions and lead to conflict. Empirical data on the relationship of trade to peace are mixed, and moreover, some evidence suggests countries at war do not necessarily trade less with each other.<ref name=Levy1998 />{{rp|150}}

====Societal-level====
* [[Diversionary foreign policy|Diversionary theory]], also known as the "scapegoat hypothesis", suggests the politically powerful may use war as a diversion or to rally domestic popular support.<ref name=Levy1998 />{{rp|152}} This is supported by literature showing out-group hostility [[Realistic conflict theory|enhances]] in-group [[Group cohesiveness|bonding]], and a significant domestic "rally effect" has been demonstrated when conflicts begin.<ref name=Levy1998 />{{rp|152–13}} However, studies examining the increased use of force as a function of need for internal political support are more mixed.<ref name=Levy1998 />{{rp|152–53}} U.S. war-time presidential popularity surveys taken during the presidencies of several recent U.S. leaders have supported diversionary theory.<ref name="MilitaryAdventurism">{{cite web| year = 2001| url = http://www.bepress.com/peps/vol7/iss3/3/| title = Peace Economics, Peace Science and Public Policy (p. 19)| access-date = 2010-02-07| url-status=dead| archive-url = https://web.archive.org/web/20110707224412/http://www.bepress.com/peps/vol7/iss3/3/| archive-date = 7 July 2011}} More recently studies (Lebow 2008, Lindemann 2010) demonstrated that striving for self-esteem (i.e. virile self images), and recognition as a Great Power or non-recognition (exclusion and punishment of great powers, denying traumatic historical events) is a principal cause of international conflict and war.
</ref>

====Individual-level====

These theories suggest that differences in people's personalities, decision-making, emotions, belief systems, and biases are important in determining whether conflicts get out of hand. For instance, it has been proposed that conflict is modulated by [[bounded rationality]] and various [[cognitive biases]],<ref name=Levy1998 />{{rp|157}} such as [[prospect theory]].<ref name=Levy1997>{{cite journal|last1=Levy|first1=Jack S.|author-link1=:de:Jack Levy|title=Prospect Theory, Rational Choice, and International Relations|journal=International Studies Quarterly|date=Mar 1997|volume=41|issue=1|pages=87–112|url=http://www.ou.edu/uschina/texts/Levy.97.ISQ.ProspectTheory.pdf|doi=10.1111/0020-8833.00034|url-status=dead|archive-url=https://web.archive.org/web/20150924075824/http://www.ou.edu/uschina/texts/Levy.97.ISQ.ProspectTheory.pdf|archive-date=24 September 2015}}</ref>

==Ethics==
[[File:1871 Vereshchagin Apotheose des Krieges anagoria.JPG|thumb|left|''[[The Apotheosis of War]]'' (1871) by [[Vasily Vereshchagin]]]]

The [[morality]] of war has been the subject of debate for thousands of years.<ref name="DeForrest">{{cite web|last=DeForrest |first=Mark Edward |title=Conclusion |url=http://www.gonzagajil.org/content/view/72/26/ |website=Just War Theory and the Recent U.S. Air Strikes Against Iraq |publisher=Gonzaga Journal of International Law |access-date=1 August 2011|url-status=dead|archive-url=https://web.archive.org/web/20100402204812/http://www.gonzagajil.org/content/view/72/26 |archive-date=2 April 2010 }}</ref>

The two principal aspects of ethics in war, according to the [[just war theory]], are ''[[jus ad bellum]]'' and ''[[jus in bello]]''.<ref>{{cite web|url=https://plato.stanford.edu/archives/spr2020/entries/war/|title=War|last=Lazar|first=Seth|date=2020-03-21|website=The Stanford Encyclopedia of Philosophy |editor-first=Edward N. |editor-last=Zalta|access-date= 2022-10-04 }}</ref>

''Jus ad bellum'' (right to war), dictates which unfriendly acts and circumstances justify a proper authority in declaring war on another nation. There are six main criteria for the declaration of a just war: first, any just war must be declared by a lawful authority; second, it must be a just and righteous cause, with sufficient gravity to merit large-scale violence; third, the just belligerent must have rightful intentions – namely, that they seek to advance good and curtail evil; fourth, a just belligerent must have a reasonable chance of success; fifth, the war must be a last resort; and sixth, the ends being sought must be proportional to means being used.<ref>{{cite web|last=Aquinas|first=Thomas|title=Part II, Question 40|url=http://ethics.sandiego.edu/Books/Texts/Aquinas/JustWar.html|archive-url=https://web.archive.org/web/20020212080058/http://ethics.sandiego.edu/Books/Texts/Aquinas/JustWar.html|url-status=dead|archive-date=12 February 2002|website=The Summa Theologica|publisher=Benziger Bros. edition, 1947|access-date=1 August 2011}}</ref><ref>{{cite web|last=Mosley|first=Alexander|title=The Jus Ad Bellum Convention|url=http://www.iep.utm.edu/justwar/|website=Just War Theory|publisher=Internet Encyclopedia of Philosophy|access-date=1 August 2011|archive-date=16 April 2010|archive-url=https://web.archive.org/web/20100416023712/http://www.iep.utm.edu/justwar/|url-status=live}}</ref>

[[File:RIAN archive 324 In besieged Leningrad.jpg|thumb|In besieged [[Leningrad]]. "Hitler ordered that Moscow and Leningrad were to be razed to the ground; their inhabitants were to be annihilated or driven out by starvation. These intentions were part of the '[[General Plan East]]'." – ''The Oxford Companion to World War II''<ref>[[I.C.B. Dear|Ian Dear]], [[M.R.D. Foot|Michael Richard Daniell Foot]] (2001). ''The Oxford Companion to World War II.'' Oxford University Press. p. 88. {{ISBN|0-19-860446-7}}.</ref>]]''[[Jus in bello]]'' (right in war), is the set of ethical rules when conducting war. The two main principles are proportionality and discrimination. Proportionality regards how much force is necessary and morally appropriate to the ends being sought and the injustice suffered. The principle of discrimination determines who are the legitimate targets in a war, and specifically makes a separation between combatants, who it is permissible to kill, and non-combatants, who it is not.<ref name="Moseley">{{cite web |last=Moseley |first=Alexander |title=The Principles of Jus in Bello |url=http://www.iep.utm.edu/justwar/ |url-status=live |archive-url=https://web.archive.org/web/20100416023712/http://www.iep.utm.edu/justwar/ |archive-date=16 April 2010 |access-date=1 August 2011 |website=Just War Theory |publisher=Internet Encyclopedia of Philosophy}}</ref> Failure to follow these rules can result in the loss of legitimacy for the just-war-belligerent.<ref name="Codevilla, Seabury 1989 304">{{cite book|last1=Codevilla|last2=Seabury|first1=Angelo|first2=Paul|title=War: Ends and Means|year=1989|publisher=Basic Books|location=New York|isbn=978-0-465-09067-9|page=[https://archive.org/details/war00paul/page/304 304]|url=https://archive.org/details/war00paul/page/304}}</ref>

The just war theory was foundational in the creation of the United Nations and in [[international law]]'s regulations on legitimate war.<ref name="DeForrest"/>

Lewis Coser, an American conflict theorist and sociologist, argued that conflict provides a function and a process whereby a succession of new equilibriums are created. Thus, the struggle of opposing forces, rather than being disruptive, may be a means of balancing and maintaining a social structure or society.<ref>Ankony, Robert C., "Sociological and Criminological Theory: Brief of Theorists, Theories, and Terms", ''CFM Research'', Jul. 2012.</ref>

==Limiting and stopping==
[[File:Washington March15 2003-02.jpg|thumb|Anti-war rally in Washington, D.C., 15 March 2003]]
{{main|Anti-war movement}}

Religious groups have long formally opposed or sought to limit war as in the [[Second Vatican Council]] document ''[[Gaudium et spes|Gaudiem et Spes]]'': "Any act of war aimed indiscriminately at the destruction of entire cities of extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation."<ref>"[https://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_cons_19651207_gaudium-et-spes_en.html Pastoral Constitution on the Church in the Modern World Gaudium et Spes Promulgated by His Holiness, Pope Paul VI on December 7, 1965] {{Webarchive|url=https://web.archive.org/web/20110411023509/https://www.vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_cons_19651207_gaudium-et-spes_en.html |date=11 April 2011 }}".</ref>

Anti-war movements have existed for every major war in the 20th century, including, most prominently, [[World War I]], [[World War II]], and the [[Vietnam War]]. In the 21st century, worldwide anti-war movements occurred in response to the United States [[War in Afghanistan (2001–2021)|invasion of Afghanistan]] and [[Iraq War|Iraq]]. Protests [[Opposition to the War in Afghanistan (2001–2021)|opposing the War in Afghanistan]] occurred in Europe, Asia, and the United States.

== Pauses ==
{{main|Ceasefire|Armistice}}

During a war, the parties may agree to pauses. A ceasefire is a stoppage of a war in which each side agrees with the other to suspend aggressive actions (often due to mediation by a third party).<ref name=":32">{{Cite book |last=Fortna |first=Virginia Page |title=Peace Time: Cease-Fire Agreements and the Durability of Peace |publisher=[[Princeton University Press]] |year=2004 |isbn=978-0-691-18795-2 |location= |pages= |oclc=1044838807}}</ref> Ceasefires may be declared as part of a formal [[treaty]] but also as part of an informal understanding between opposing forces.<ref name=":02">{{Citation |last=Forster |first=Robert A. |title=Ceasefires |date=2019 |pages=1–8 |editor-last=Romaniuk |editor-first=Scott |publisher=Springer |language=en |doi=10.1007/978-3-319-74336-3_8-2 |isbn=978-3-319-74336-3 |s2cid=239326729 |editor2-last=Thapa |editor2-first=Manish |editor3-last=Marton |editor3-first=Péter |encyclopedia=The Palgrave Encyclopedia of Global Security Studies}}</ref> A ceasefire can be temporary with an intended end date or may be intended to last indefinitely. A ceasefire is distinct from an [[armistice]] in that the armistice is a formal end to a war whereas a ceasefire may be a temporary stoppage.<ref>{{cite web | last=Fortna | first=Page | title=Ceasefires are fragile: Can Israel and Hamas find peace? | website=Good Authority | date=2025-01-17 | url=https://goodauthority.org/news/good-to-know-what-is-a-ceasefire-israel-hamas/ | access-date=2025-05-03}}</ref>

The immediate goal of a ceasefire is to stop violence, but the underlying purposes of ceasefires vary. Ceasefires may be intended to meet short-term limited needs (such as providing humanitarian aid), manage a conflict to make it less devastating, or advance efforts to peacefully resolve a dispute.<ref name=":22">{{Cite journal |last1=Clayton |first1=Govinda |last2=Nygård |first2=Håvard Mokleiv |last3=Rustad |first3=Siri Aas |last4=Strand |first4=Håvard |date=2023 |title=Ceasefires in Civil Conflict: A Research Agenda |journal=Journal of Conflict Resolution |language=en |volume=67 |issue=7–8 |pages=1279–1295 |doi=10.1177/00220027221128300 |issn=0022-0027 |s2cid=252793375 |doi-access=free |hdl-access=free |hdl=20.500.11850/576568}}</ref> An actor may not always intend for a ceasefire to advance the peaceful resolution of a conflict but instead give the actor an upper hand in the conflict (for example, by re-arming and repositioning forces or attacking an unsuspecting adversary), which creates [[Bargaining model of war|bargaining problems]] that may make ceasefires less likely to be implemented and less likely to be durable if implemented.<ref name=":33">{{Cite book |last=Fortna |first=Virginia Page |title=Peace Time: Cease-Fire Agreements and the Durability of Peace |publisher=[[Princeton University Press]] |year=2004 |isbn=978-0-691-18795-2 |location= |pages= |oclc=1044838807}}</ref><ref name=":22" /><ref>{{Cite book |last=Sosnowski |first=Marika |url=https://www.cambridge.org/core/books/redefining-ceasefires/AEDBE40203BCD110435E69E19D931BF3 |title=Redefining Ceasefires: Wartime Order and Statebuilding in Syria |date=2023 |publisher=[[Cambridge University Press]] |isbn=978-1-009-34722-8 |doi=10.1017/9781009347204}}</ref>

The durability of ceasefire agreements is affected by several factors, such as demilitarized zones, withdrawal of troops and third-party guarantees and monitoring (e.g. [[peacekeeping]]). Ceasefire agreements are more likely to be durable when they reduce incentives to attack, reduce uncertainty about the adversary's intentions, and when mechanisms are put in place to prevent accidents from spiraling into conflict.<ref name=":33" />

==See also==
* [[Outline of war]]
{{-}}

==Notes==
{{notelist}}
==References==
{{Reflist|30em}}

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{{refend}}

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