WAR may be defined as the condition which prevails while groups are contending by arms. War is at the same time an exceptional condition, a phenomenon of intergroup social psychology, a species of contention, and a species of violence. While each of these aspects of war suggests an approach to its study, war must not be identified with any one of them. We may throw light on war by studying other exceptional legal conditions like civil litigation, criminal trials, martial law, reprisals, but we must not identify them with war as have some writers in characterizing all legal coercion as war. So also we must not assume that all relations between sovereign groups are war, or that all contention or that all violence is war. Such assumptions, frequently made, render the control of war hopeless. The anarchist striving to eliminate all legal coercion, the isolationist striving to eliminate all intergroup relations, the idealist trying to eliminate all contentions, and the extreme pacifist trying to eliminate all violence are engaged in a hopeless task. On the other hand, it is possible that appropriate modifications of international law and procedure, of national attitudes and ideals, of social and economic conditions, and of the methods by which governments keep themselves in power may prevent the recurrence of war. A detailed analysis has been made of the circumstances preceding the Moslem wars of conquest in the seventh century, the Crusades beginning in the eleventh century, the Hundred Years' War beginning in the fourteenth century, the Thirty Years' War of the seventeenth century, the French Revolutionary and Napoleonic Wars and the World War. Different as were many of their circumstances, all of these wars, scattered over I300 years, exhibit idealistic, psychological, political, and juridical causes. Let us examine each of these four causes more closely. (i) War among civilized peoples has meant great cost, risk, and sacrifice both to civilians and participants. Only widespread belief in an ideal more important than life itself can induce most men and women to carry that burden. In propor-