Evolutionary anthropologists seek to understand the evolution of warfare across multiple timescales, from the roots of warfare in the intergroup aggression of our primate ancestors, to the causes of warfare among contemporary societies today. While warfare remains a contentious subject, considerable evidence supports the view that warfare is a strategy by which coalitions of males cooperate to acquire and defend resources necessary for reproduction. This strategy is not the result of a single “instinct” for war, but is instead an emergent property resulting from evolved psychological mechanisms (such as xenophobia and parochial altruism). These mechanisms are sensitive to ecological and social conditions, such that the prevalence and patterns of warfare vary according to subsistence strategies, military technology, cultural institutions, and political and economic relations. When economic conditions enable intergroup relations to change from zero-sum to positive-sum games, peaceful intergroup relations can emerge.