War is a state of armed conflict between societies. It is generally characterized by extreme aggression, destruction, and mortality, using regular or irregular military forces. An absence of war is usually called "peace". Total war is warfare that is not restricted to purely legitimate military targets, and can result in massive civilian or other non-combatant casualties.

While some scholars see war as a universal and ancestral aspect of human nature, others argue it is a result of specific socio-cultural or ecological circumstances.[2]

The deadliest war in history, in terms of the cumulative number of deaths since its start, is the Second World War, from 1939 to 1945, with 60–85 million deaths, followed by the Mongol conquests which was greater than 41 million. Proportionally speaking, the most destructive war in modern history is the War of the Triple Alliance, which took the lives of over 60% of Paraguay's population, according to Steven Pinker. In 2013 war resulted in 31,000 deaths, down from 72,000 deaths in 1990.[4] In 2003, Richard Smalley identified war as the sixth (of ten) biggest problem facing humanity for the next fifty years. 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 prisoners of war or civilians. Another byproduct of some wars is the prevalence of propaganda by some or all parties in the conflict, and increased revenues by weapons manufacturers.


The English word war derives from the late Old English (circa.1050) words wyrre and werre, from Old French werre (also guerre as in modern French), in turn from the Frankish *werra, ultimately deriving from the Proto-Germanic *werzō 'mixture, confusion'. The word is related to the Old Saxon werran, Old High German werran, and the German verwirren, meaning “to confuse”, “to perplex”, and “to bring into confusion”.[6] In German, the equivalent is Krieg (from Proto-Germanic *krīganą 'to strive, be stubborn'); the Spanish, Portuguese, and Italian term for "war" is guerra, derived like the Old French term from the Germanic word. Etymologic legend has it that the Romanic peoples adopted a foreign, Germanic word for "war", to avoid using the Latin bellum, because, when sounded, it tended to merge with the sound of the word bello ("beautiful").

The scholarly study of war is sometimes called polemology (American English: /ˌpɑːləˈmɑːləi/ pah-lə-MAHL-ə-jee), from the Greek polemos, meaning "war", and -logy, meaning "the study of".


War must entail some degree of confrontation using weapons and other military technology and equipment by armed forces employing military tactics and operational art within a broad military strategy subject to military logistics. Studies of war by military theorists throughout military history have sought to identify the philosophy of war, and to reduce it to a military science. Modern military science considers several factors before a national defence policy is created to allow a war to commence: the environment in the area(s) of combat operations, the posture national forces will adopt on the commencement of a war, and the type of warfare troops will be engaged in.

  • Asymmetric warfare is a conflict between two populations of drastically different levels of military capability or size.
  • Biological warfare, or germ warfare, is the use of weaponized biological toxins or infectious agents such as bacteria, viruses, and fungi.
  • Chemical warfare involves the use of weaponized chemicals in combat. Poison gas as a chemical weapon was principally used during World War I, and resulted in over a million estimated casualties, including more than 100,000 civilians.


The earliest recorded evidence of war belongs to the Mesolithic cemetery Site 117, which has been determined to be approximately 14,000 years old. About forty-five percent of the skeletons there displayed signs of violent death. Since the rise of the state some 5,000 years ago, military activity has occurred over much of the globe. The advent of gunpowder and the acceleration of technological advances led to modern warfare. According to Conway W. Henderson, "One source claims that 14,500 wars have taken place between 3500 BC and the late 20th century, costing 3.5 billion lives, leaving only 300 years of peace (Beer 1981: 20)."[7]

In War Before Civilization, Lawrence H. Keeley, a professor at the University of Illinois, says approximately 90–95% of known societies throughout history engaged in at least occasional warfare,[8] and many fought constantly.[9]

Keeley describes several styles of primitive combat such as small raids, large raids, and massacres. All of these forms of warfare were used by primitive societies, a finding supported by other researchers.[10] 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.

William Rubinstein wrote "Pre-literate societies, even those organised in a relatively advanced way, were renowned for their studied cruelty...'archaeology yields evidence of prehistoric massacres more severe than any recounted in ethnography [i.e., after the coming of the Europeans].'"[11]

In Western Europe, since the late 18th century, more than 150 conflicts and about 600 battles have taken place.[12] During the 20th century, war resulted in a dramatic intensification of the pace of social changes, and was a crucial catalyst for the emergence of the Left as a force to be reckoned with.

Recent rapid increases in the technologies of war, and therefore in its destructiveness (see mutual assured destruction), have caused widespread public concern, and have in all probability forestalled, and may altogether prevent the outbreak of a nuclear World War III. At the end of each of the last two World Wars, concerted and popular efforts were made to come to a greater understanding of the underlying dynamics of war and to thereby hopefully reduce or even eliminate it altogether. These efforts materialized in the forms of the League of Nations, and its successor, the United Nations.

Shortly after World War II, as a token of support for this concept, most nations joined the United Nations. During this same post-war period, with the aim of further delegitimizing war as an acceptable and logical extension of foreign policy, most national governments also renamed their Ministries or Departments of War as their Ministries or Departments of Defense, for example, the former U.S. Department of War was renamed as the U.S. Department of Defense.

In 1947, in view of the rapidly increasingly destructive consequences of modern warfare, and with a particular concern for the consequences and costs of the newly developed atom bomb, Albert Einstein famously stated, "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."[14]

Mao Zedong urged the socialist camp not to fear nuclear war with the United States since, even if "half of mankind died, the other half would remain while imperialism would be razed to the ground and the whole world would become socialist."[15]

The Human Security Report 2005 documented a significant decline in the number and severity of armed conflicts since the end of the Cold War in the early 1990s. However, the evidence examined in the 2008 edition of the Center for International Development and Conflict Management's "Peace and Conflict" study indicated the overall decline in conflicts had stalled.[16]

Largest by death toll

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 million, surpasses all other war-death-tolls.[17]

60.7–84.61939–1945World War II (see World War II casualties) [18]
6013th centuryMongol Conquests (see Mongol invasions and Tatar invasions)[19][20]
401850–1864Taiping Rebellion (see Dungan revolt)[21]
391914–1918World War I (see World War I casualties)[22]
36755–763An Shi Rebellion (death toll uncertain)[23]
251616–1662Qing dynasty conquest of Ming dynasty[17]
201937–1945Second Sino-Japanese War[24]
201370–1405Conquests of Tamerlane[25][26]
161862–1877Dungan revolt
5–91917–1922Russian Civil War and Foreign Intervention[28]


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.

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[12]

During World War II, research conducted by US Army Brigadier General S.L.A. Marshall found, on average, 15% to 20% of American riflemen in WWII combat fired at the enemy. In Civil War Collector’s Encyclopedia, F.A. Lord notes that of the 27,574 discarded muskets found on the Gettysburg battlefield, nearly 90% were loaded, with 12,000 loaded more than once and 6,000 loaded 3 to 10 times. These studies suggest most military personnel resist firing their weapons in combat, that – as some theorists argue – human beings have an inherent resistance to killing their fellow human beings. Swank and Marchand’s WWII 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.

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[12]

Additionally, it has been estimated anywhere from 18% to 54% of Vietnam war veterans suffered from Post Traumatic Stress Disorder.

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.[30] 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 million European military personnel who were mobilized in World War I, 8 million were killed, 7 million were permanently disabled, and 15 million were seriously injured.[31]

During Napoleon's retreat from Moscow, more French military personnel died of typhus than were killed by the Russians.[32] Of the 450,000 soldiers who crossed the Neman on 25 June 1812, less than 40,000 returned. More military personnel were killed from 1500–1914 by typhus than from military action.[33] 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 died of disease or were 'missing'.

It is estimated that between 1985 and 1994, 378,000 people per year died due to war.[35]

On civilians

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). During the Thirty Years' War in Europe, the population of the Holy Roman Empire was reduced by 15 to 40 percent.[37][38] 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.

Most estimates of World War II casualties indicate around 60 million people died, 40 million of which were civilians.[39] Deaths in the Soviet Union were around 27 million.[40] 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.[41]

On the economy

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.

Typically, war becomes intertwined with the economy and many wars are partially or entirely based on economic reasons. Some economists believe war can stimulate a country's economy (high government spending for World War II is often credited with bringing the USA out of the Great Depression by most Keynesian economists) but in many cases, such as the wars of Louis XIV, the Franco-Prussian War, and World War I, warfare primarily results in damage 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.

World War II

The financial cost of World War II is estimated at about a trillion U.S. dollars worldwide, making it the most costly war in capital as well as lives.[43][44] The Great Depression of the 1930s ended as nations increased their production of war materials to serve the war effort.[46]

By the end of the war, 70% of European industrial infrastructure was destroyed.[47] Property damage in the Soviet Union inflicted by the Axis invasion was estimated at a value of 679 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, 40,000 mi (64,374 km) of railroad, 4100 railroad stations, 40,000 hospitals, 84,000 schools, and 43,000 public libraries.

On the arts

War leads to forced migration causing potentially large displacements of population. Among forced migrants there are usually relatively large shares of artists and other types of creative people, causing so the war effects to be particularly harmful for the country’s creative potential in the long-run.[48] War also has a negative effect on an artists’ individual life-cycle output.[49]

In war, cultural institutions, such as libraries, can become "targets in themselves; their elimination was a way to denigrate and demoralize the enemy population." The impact such destruction can have on a society is important because "in an era in which competing ideologies fuel internal and international conflict, the destruction of libraries and other items of cultural significance is neither random nor irrelevant. Preserving the world’s repositories of knowledge is crucial to ensuring that the darkest moments of history do not endlessly repeat themselves."


Entities deliberately 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.[51]


Fried defines war aims as "the desired territorial, economic, military or other benefits expected following successful conclusion of a war".[52]


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[53] - may have more tangible expression ("conquest restores prestige, annexation increases power").

Explicit/implicit aims:

  • Explicit war aims may involve published policy decisions.
  • Implicit war aims[54] can take the form of minutes of discussion, memoranda and instructions.

Positive/negative aims:

  • "Positive war aims" cover tangible outcomes.
  • "Negative war aims" forestall or prevent undesired outcomes.[55]

War aims can change in the course of conflict and may eventually morph into "peace conditions"[56] - the minimal conditions under which a state may cease to wage a particular war.

Ongoing conflicts

There are currently dozens of ongoing armed conflicts around the world, the deadliest of which is the Syrian Civil War.

Limiting and stopping

Religious groups have long formally opposed or sought to limit war as in the Second Vatican Council document 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."[57]

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 invasion of Afghanistan and Iraq. Protests opposing the War in Afghanistan occurred in Europe, Asia, and the United States. Organizations like Stop the War Coalition, based in the United Kingdom, worked on campaigning against the war.[59]

The Mexican Drug War, with estimated casualties of 40,000 since December 2006, has recently faced fundamental opposition.[60] In 2011, the movement for peace and justice has started a popular middle-class movement against the war. It won the recognition of President Calderon, who began war.[61]

Theories for motivation

There is no scholarly agreement on which are the most common motivations for war.[63] Carl von Clausewitz said, 'Every age had its own kind of war, its own limiting conditions, and its own peculiar preconceptions.'

Psychoanalytic psychology

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." Thus war can sometimes be a means by which man's own frustration at his inability to master his own self is expressed and temporarily relieved via his unleashing of destructive behavior upon others. In this destructive scenario, these others are made to serve as the scapegoat of unspoken and subconscious frustrations and fears.

Other psychoanalysts such as E.F.M. Durban and John Bowlby have argued human beings are inherently violent. This aggressiveness is fueled by displacement and projection where a person transfers his or her grievances into bias and hatred against other races, religions, nations or 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. 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. 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. 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 and 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.

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, April 18, 1946[64]


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 resources; the other sees human warfare as a more ancient practice derived from common animal tendencies, such as territoriality and sexual competition.

The latter school argues that since warlike behavior patterns are found in many primate species such as chimpanzees,[65] as well as in many ant species,[66] 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.

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.[68] He also argued that in order to have credible 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").

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".

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. 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 of the Malay peninsula. Bobbi S. Low has observed correlation between warfare and education, noting societies where warfare is commonplace encourage their children to be more aggressive.[69]


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 resources and for wealth. War has also been linked to economic development by economic historians and development economists studying state-building and fiscal capacity.[70] 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 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. Some centrist, capitalist, world leaders, including Presidents of the United States and U.S. Generals, expressed support for an economic view of war.


The 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 the free market and class system. Part of the theory is that war will disappear once a world revolution, over-throwing free markets and class systems, has occurred. Marxist philosopher Rosa Luxemburg theorized that imperialism was the result of capitalist countries needing new 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.[71]


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


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, spoke:

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.

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.

Youth bulge

According to 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 rates as high as 4–8 children per woman with a 15–29-year delay.[72][73]

Heinsohn saw both past "Christianist" European colonialism and imperialism, as wellas today's Islamist civil unrest and terrorism as results of high birth rates producing youth bulges.[74] 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,[75] and the affect of economic depression upon the largest German youth cohorts ever in explaining the rise of Nazism in Germany in the 1930s. The 1994 Rwandan Genocide has also been analyzed as following a massive youth bulge.[76]

Youth bulge theory has been subjected to statistical analysis by the World Bank,[77] Population Action International,[78] and the Berlin Institute for Population and Development.[79] Youth bulge theories have been criticized as leading to racial, gender and age discrimination.[80]


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.[81] Under a Game theory approach, rationalist theories posit all actors can bargain, would be better off if war did not occur, and likewise seek to understand why war nonetheless reoccurs. In "Rationalist Explanations for War", James Fearon examined three rationalist explanations for why some countries engage in war:

  1. Issue Indivisibilities
  2. Incentives to Misrepresent or Information Asymmetry
  3. Commitment Problems[81]

"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."[82] Thirdly, bargaining may fail due to the states' inability to make credible commitments.

Within the rationalist tradition, some theorists have suggested individuals engaged in war suffer a normal level of cognitive bias,[6] but are still "as rational as you and me".[6] According to philosopher Iain King, "Most instigators of conflict overrate their chances of success, while most participants underrate their chances of injury...."[6] King asserts that "Most catastrophic military decisions are rooted in GroupThink" which is faulty, but still rational.[6]

The rationalist theory focused around bargaining is currently under debate. The Iraq War proved to be an anomaly that undercuts the validity of applying rationalist theory to some wars.[6]

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.

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.[87]:143

System-level theories

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.[87]:145

Within the realist school as represented by scholars such as Henry Kissinger and Hans Morgenthau, and the neorealist school represented by scholars such as Kenneth Waltz and John Mearsheimer, two main sub-theories are:

  1. 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."[87]:147 However, evidence has shown power polarity is not actually a major factor in the occurrence of wars.[87]:147–148
  2. 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 preemptively suppress them.[87]:148 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.[87]:148

The two theories are not mutually exclusive, and may be used to explain disparate events according to the circumstance.[87]:148

Liberalism at it relates to international relations emphasizes factors such as trade, and it's role in disincentivizing conflict which will damage economic relations. Realists respond that military force may sometimes be at least as effective as trade at achieving economic benefits, especially historically if not as much today.[87]:149 Furthermore, trade relations which result in a high level of dependency, may escalate tensions and lead to conflict.[87]:150 Empirical data on the relationship of trade to peace are mixed, and moreover, some evidence suggests countries at war don't necessarily trade less with each other.[87]:150

Societal-level theories

  • Diversionary theory, also known as the "scapegoat hypothesis", suggests the politically powerful may use war to as a diversion or to rally domestic popular support.[87]:152 This is supported by literature showing outgroup hostility enhances ingroup bonding, and a significant domestic "rally effect" has been demonstrated when conflicts begin.[87]:152–153 However, studies examining the increased use of force as a function of need for internal political support are more mixed.[87]:152–153 U.S. war-time presidential popularity surveys taken during the presidencies of several recent U.S. leaders have supported diversionary theory.[88]

Individual-level theories

These theories suggest differences in people's personalities, decision-making, emotions, belief systems, and biases are important in determining whether conflicts get out of hand.[87]:157 For instance, it has been proposed that conflict is modulated by bounded rationality and various cognitive biases,[87]:157 such as prospect theory.[90]


The morality of war has been the subject of debate for thousands of years.[92]

The two principle aspects of ethics in war, according the Just War Theory are jus ad bellum and Jus in bello.[6][93]

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.[94][95]

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 who it is not. Failure to follow these rules can result in the loss of legitimacy for the just-war-belligerent.[93]

Just War Theory was foundational in the creation of the United Nations and in International Law's regulations on legitimate war.[92]

Fascism, and the ideals it encompasses, such as Pragmatism, racism, and social Darwinism, hold that violence is good.[96] Pragmatism holds that war and violence can be good if it serves the ends of the people, without regard for universal morality. Racism holds that violence is good so that a master race can be established, or to purge an inferior race from the earth, or both. Social Darwinism asserts that violence is sometimes necessary to weed the unfit from society so civilization can flourish. These are broad archetypes for the general position that the ends justify the means. Lewis Coser, U.S. conflict theorist and sociologist, argued 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.

General reference
War-related lists