War is a state of open or covert conflict between nations or other organised armed groups (such as rebels) which can involve fighting or the threat of violence. War may refer to actual battles between opposing forces, or a more generalized hostility between peoples and ideologies. It is often used to refer to political and military activities that are aimed at achieving some strategic objective, such as the retaking of territory or the control of resources. The study of war is important because its theory shapes expectations and influences human behaviour. Different schools of thought have developed theories about its causes, conduct and prevention.

The concept of war is complex and varied. The term is defined differently by many writers: Cicero defines it as “contention by force”; Hugo Grotius adds that it is a “state of contending parties”; Thomas Hobbes describes it as a “struggle for power”; Denis Diderot calls it a “convulsive disease of the body politic”; and Karl von Clausewitz writes that it is ‘the continuation of politics by other means’.

While the concept of war is diverse, all definitions contain a common element in that they imply that a war must be justified by a set of criteria. These include a reasonable chance of success, the good that will be achieved must outweigh the bad, and it should only be undertaken as a last resort. The application of these criteria is difficult, however, as wars are often unforeseeable.

In practice, war is a complex interaction between belligerents over a long period of time. Decisions are made at the tactical, strategic and national levels, and each side must evaluate its options in the light of possible reactions by the other. Moreover, as the belligerents interact their objectives and ideas will change. Consequently, there is no such thing as a ‘perfect war’.

Philosophical debates have also centred on the role, if any, of morality in war. Some have claimed that morality is necessarily discarded in war, including Christian thinkers such as Augustine; others have sought to remind warriors of their duty and have encouraged them to follow various strictures to remain sensitive to moral ends in the face of the horrors of war.

Sociological studies of war have noted that those who go to fight are often regarded as a special type of person, with specific social connections and obligations to the community that they leave behind. Hence some theorists have tried to construct a theory of humanity that accounts for this, in particular by evoking John Donne’s rephrasing of Aristotle’s idea that “no man is an island”. The more organic schools of thought, in contrast, have argued that this is an unrealistically atomistic conception of human nature and that any theoretical construction of human nature and war must take into account the social structures that form it. In this context the’social contract’ argument is often cited.