IHE problem of war and national security, at one time conceived of as the province of almost anyone but the peace-minded international relationist, has come increasingly to occupy scholars and researchers. Arnold J. Toynbee, whose major concern is the philosophy of history, preserves a lively interest in international politics and particularly in the problem of war, the principles of foreign policy, and the quest for an applicable body of theory concerning international society. With the publication of the last four volumes of his famed A Study of History, it may be appropriate to call attention to the other side of his work, especially as he brings to the discussion a clarity, simplicity, and concreteness refreshing by contrast with the pompous tautologies of much of modern scholarship. This article reviews Mr. Toynbee's contribution to knowledge on the first of the problems mentioned, namely, war and national security. It seeks to present his conception of the crisis in modern war, social factors underlying the transformation of warfare, and prevailing theories on the nature and inevitability of war.