War
"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". According to philosopher Iain King, "Most instigators of conflict overrate their chances of success, while most participants underrate their chances of injury...." King asserts that "Most catastrophic military decisions are rooted in groupthink" which is faulty, but still rational. 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.

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.: 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.: 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:

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.": 147  However, evidence has shown power polarity is not actually a major factor in the occurrence of wars.: 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.: 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.: 148 
The two theories are not mutually exclusive and may be used to explain disparate events according to the circumstance.: 148  Liberalism as it relates to international relations emphasizes factors such as 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.: 149  Furthermore, trade relations which result in a high level of dependency may escalate tensions and lead to conflict.: 150  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.: 150

Societal-level: 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.: 152  This is supported by literature showing out-group hostility enhances in-group bonding, and a significant domestic "rally effect" has been demonstrated when conflicts begin.: 152–13  However, studies examining the increased use of force as a function of need for internal political support are more mixed.: 152–53  U.S. war-time presidential popularity surveys taken during the presidencies of several recent U.S. leaders have supported diversionary theory.

Individual-level: 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.: 157  For instance, it has been proposed that conflict is modulated by bounded rationality and various cognitive biases,: 157  such as prospect theory.

Ethics: The morality of war has been the subject of debate for thousands of years.
The two principal aspects of ethics in war, according to the just war theory, are jus ad bellum and jus in bello.
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.

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. Failure to follow these rules can result in the loss of legitimacy for the just-war-belligerent.
The just war theory was foundational in the creation of the United Nations and in international law's regulations on legitimate war.
Lewis Coser, an American 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.

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."
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.

Pauses: During a war, brief pauses of violence may be called for, and further agreed to – ceasefire, temporary cessation, humanitarian pauses and corridors, days of tranquility, de-confliction arrangements. There are a number of disadvantages, obstacles and hesitations against implementing such pauses such as a humanitarian corridor. Pauses in conflict can also be ill-advised, for reasons such as "delay of defeat" and the "weakening of credibility". Natural causes for a pause may include events such as the 2019 coronavirus pandemic.

See also: Outline of war
Grey-zone (international relations)

Notes: 

References: 

Bibliography: 

External links: 
An Interactive map of all the battles fought around the world in the last 4,000 years
Timeline of wars on Histropedia

 War zone safety travel guide from Wikivoyage