From a behavioral ecology perspective, all forms of warfare are instances of collective aggression perpetrated predominantly by coalitions of young men. Such coalitions are manifestations of cross-cultural sex differences in aggressive behavior and may be conceptualized as a form of intrasexual competition, occasionally to obtain mates, but more often to acquire resources for the attraction and retention of mates. All societies have young males, yet wars are discrete events that can take place even after long periods of peace. Therefore, an additional factor is needed to explain the episodic nature of the phenomenon. We have proposed (Mesquida and Wiener, 1996) that the most reliable factor in explaining episodes of coalitional aggression is the relative abundance of young males. In this article, we present additional evidence to that effect. The ratio of the number of men ages 15 to 29 years of age versus men 30 and older in a population appears to be associated with the occurrence and severity of conflicts as measured by the number of war casualties. A series of analyses of demographic and war casualty data indicates that the relative prevalence of young men consistently accounts for more than one third of the variance in severity of conflicts.