Warfare is enormously destructive, and yet countries regularly initiate armed conflict against one another. Even more surprisingly, wars are often quite popular with citizens who stand to gain little materially and may lose much more. This paper presents a model of warfare as the result of domestic political calculations. When incumbents have an edge in fighting wars, they may start wars even if those wars run counter to their country's interests. Challengers are particularly likely to urge aggression when they are unlikely to come into power and when the gains from coming to power are large. Leaders who start wars will naturally try to create hatred by emphasizing the threat and despicable character of the rival country. Wars will be more common in dictatorships than in democracies both because dictators have stronger incentives to stay in power and because they have greater control over the media.