Students of international politics have devoted enormous effort to understanding the causes of war. There exists a rich literature on the correlates of war; on the war-proneness of various types of international systems, or of various types of regimes; on the influence of societal, domestic political, economic, bureaucratic, or military factors; on the impact of belief systems, operational codes, processes of cognition, affect, and personality; and on the effects of decision-making processes on decisions themselves. Can we plausibly say that the causes of war are well understood? Certainly not as well as we would like. Wars are, at present, largely unpredictable events. Yet few (if any) wars are entirely mysterious, at least in hindsight. We understand enough about the conditions under which wars are more or less likely to enable us to recognize with some degree of reliability moments of particular danger. In June 1914, hardly anyone suspected that the assassination of Austrian Archduke Francis Ferdinand would lead to a general war less than five weeks later; yet, as early as 1912, the common wisdom held that a major European war was likely within a few years.

Ideally, students of international conflict would like to specify the set of necessary and sufficient conditions for war. This goal is presently out of reach, and may well be unobtainable in principle.