War
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.

Evolutionary: 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, as well as in many ant species, 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. 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.

Economic: 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. 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.

Marxist: 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 capitalism. Marxist economists Karl Kautsky, Rosa Luxemburg, Rudolf Hilferding and Vladimir Lenin 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.

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

Malthusian: 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:

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. The violent herder–farmer conflicts in Nigeria, Mali, Sudan and other countries in the Sahel region have been exacerbated by land degradation and population growth.

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

Cultural: 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: 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 – from about 1500 onward – 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.
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.

Rationalist: 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. Under one 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. 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
"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." Thirdly, bargaining may fail due to the states' inability to make credible commitments.
Within the rationalist tradition, some theorists have suggested that individuals engaged in war suffer a normal level of cognitive bias, but are still "as rational as you and me".