War is a human enterprise that entails a struggle to overcome opposing forces, whether they are military or ideological. The Oxford English Dictionary explains the term thus: “War is active hostility between opposing parties, whether it be in the form of direct assault or indirect aggression.” The latter point is particularly relevant to contemporary conflicts in which terrorists, spies, and other adversaries employ propaganda as part of their campaigns to sow divisions amongst civilian populations.

The ethical and philosophical problems associated with war encompass a range of themes. For example, questions about what is permissible in warfare often focus on the principles of discrimination and proportionality. Others address the nature of the causes of war, or debate its moral legitimacy. Some claim that a war cannot be just unless its aims are sufficiently compelling, while others assert that a just war is one that has the right mix of good and bad effects. Still others examine whether a war may be justified at all, while many more explore the ways in which it might be made less likely to occur.

Some theories of war seek to prevent it before it occurs through means such as promoting international law, dispelling superstitions about the efficacy of armaments (cf. Nietzsche), educating the masses about the dangers of war (cf. John Donne), and ensuring that governments do not exceed certain limits of power and/or the capacity to manufacture weapons. Other theorists focus on improving states’ rationality by eliminating misperceptions and irrational fears, and by clarifying the full costs of war and its possible outcomes.

Still others argue that the problem of war is endemic to the structure of society itself, and thus any theory of man’s natural propensity for violence must take account of the social context in which he lives. In this vein, some critics of Hobbes contend that the notion of the isolated individual pitted against his fellow man is ill-suited to understanding human life, while others invoke Aristotle’s view that man is a political animal.

A number of theorists, most notably Carl von Clausewitz and Winston Churchill, emphasize the centrality of the national state to human affairs, with their primary concern being to preserve the political strength of nation-states in order to ensure their security against hostile foreign powers. They assert that only those nations with the financial and military resources to sustain a substantial global military presence can serve as a counterweight to powerful and aggressive regional states. Those who take this view tend to support economic sanctions against regional aggressors, and they also favor the development of an international police force with the power to enforce those sanctions militarily as needed. They are not, however, as sanguine about the prospects for conflict resolution through such mechanisms as mediation and conciliation. They are skeptical of the possibility of reaching a comprehensive treaty that would permanently bind all great powers to restrain their ambitions and prevent future world wars. They do not consider the threat of nuclear war to be sufficient for this purpose, as they believe that even a limited exchange of nuclear devices could produce devastating consequences.