War, in the popular sense, a conflict between political groups involving hostilities of considerable duration and magnitude. In the usage of social science, certain qualifications are added. Sociologists usually apply the term to such conflicts only if they are initiated and conducted in accordance with socially recognized forms. They treat war as an institution recognized in custom or in law. Military writers usually confine the term to hostilities in which the contending groups are sufficiently equal in power to render the outcome uncertain for a time. Armed conflicts of powerful states with isolated and powerless peoples are usually called pacifications, military expeditions, or explorations; with small states, they are called interventions or reprisals; and with internal groups, rebellions or insurrections. Such incidents, if the resistance is sufficiently strong or protracted, may achieve a magnitude that entitles them to the name “war.”

In all ages war has been an important topic of analysis. In the latter part of the 20th century, in the aftermath of two World Wars and in the shadow of nuclear, biological, and chemical holocaust, more was written on the subject than ever before. Endeavours to understand the nature of war, to formulate some theory of its causes, conduct, and prevention, are of great importance, for theory shapes human expectations and determines human behaviour. The various schools of theorists are generally aware of the profound influence they can exercise upon life, and their writings usually include a strong normative element, for, when accepted by politicians, their ideas can assume the characteristics of self-fulfilling prophecies.

Evolution Of Theories Of WarReflecting changes in the international system, theories of war have passed through several phases in the course of the past three centuries. After the ending of the wars of religion, about the middle of the 17th century, wars were fought for the interests of individual sovereigns and were limited both in their objectives and in their scope. The art of maneuver became decisive, and analysis of war was couched accordingly in terms of strategies. The situation changed fundamentally with the outbreak of the French Revolution, which increased the size of forces from small professional to large con armies and broadened the objectives of war to the ideals of the revolution, ideals that appealed to the masses who were subject to conion. In the relative order of post-Napoleonic Europe, the mainstream of theory returned to the idea of war as a rational, limited instrument of national policy. This approach was best articulated by the Prussian military theorist Carl von Clausewitz in his famous classic On War (1832–37).