War, as a state of organized collective hostility between a sovereign body and its subjects, has been the subject of many intellectual explorations throughout history. These explorations have often reflected broader philosophical issues concerning the nature of determinism and freedom. For example, the definition of “war” itself is a philosophical issue because it determines the entities who may declare and engage in such conflict. Defining war also shapes how we think about it, for instance, whether we regard it as actual or threatened clashes of arms or as metaphorical clashes of values and the ways in which those values are expressed.

In addition, any exploration of war must confront the central issues of political philosophy – especially conceptions of sovereignty and the responsibilities of the state. These are often intertwined with questions of morality. As a consequence, theories of war often unwittingly delve into related philosophical issues concerning the morality of violence and the limits of human rights.

Contemporary theories of war are broadly divided into two major categories. One school attributes war to certain innate biological and psychological factors or drives, as viewed by ethologists who draw analogies from animal behavior and by psychologists and psychoanalysts. This school includes optimists who think that the causes of war can be understood and prevented, as well as pessimists who believe it is impossible to stop or even control war.

Another school views war as a necessary and inevitable aspect of international politics, as viewed by Realists who subscribe to the philosophy that power in the world flows from a hierarchy of competing states. The most significant and powerful Realist theory on the causes of war is called the offense-defense theory, which argues that war is more likely when governments believe that it is easy to conquer their opponents.

The high costs of war (both in terms of life and economics) make countries very reluctant to fight, unless they feel that the alternatives are worse. This insight, combined with the insights of game theory, helps explain why it is so difficult to stop or even control wars once they begin. It also explains why, once they are underway, nations try to keep their fighting contained and at a minimum avoid escalating it into full-scale warfare, in order to minimize the chance that it could become nuclear.