This article surveys the history since the Enlightenment of the controversy over the origins and functions of warfare, focusing on the question of whether war is caused by nature or nurture. In the earlier literature (before 1950) five positions are distinguished. (1) The Hobbesian thesis: war is part of human nature and serves both the internal function of solidarity and the external function of maintaining the balance of power. (2) The Rousseauean thesis: war is not in human nature but was invented by states for the functions mentioned above. (3) The Malthusian thesis: war serves the grand function of reducing population, quite apart from its conscious proximate functions. (4) The Spencerian thesis: a combination of Hobbes and Malthus - war serves the grand function of human evolution. (5) The cultural anthropologists’ thesis: an extreme version of Rousseau - war is a dysfunctional historical accident. Most of the article is devoted to recent controversy, distinguishing three major theories: (1) sociobiology, an updated version of the Spencerian thesis; (2) cultural ecology, an updated version of the cultural-anthropological thesis, combining Rousseau and Malthus; (3) cultural Darwinism, which holds that the process of cultural evolution mimics natural selection. The last theory is favored here. It implies that warfare has no grand functions, either sociobiological or ecological. War is neither nature nor nurture, but nurture imitating nature. Hobbes was right in thinking war has always been around; Rousseau was right to think primitive warfare was not the same thing as the wars of states.