Warning about the dire effects of climate change on armed conflict is a recent variation of a scenario that has been promoted by environmental pessimists for over two centuries. The essence is that human activities lead to resource scarcities that in turn will generate famine, pestilence, and war. This essay reviews three stages of the argument: first, the original Malthusian thesis focused on food production. Second, the broader neo-Malthusian concern from the 1970s about limits to growth and developing scarcities in a range of necessities. And recently, the specter of climate change. In each phase, the Malthusians have met firm opposition from environmental optimists, who argue that emerging scarcities can be countered by human ingenuity, technological progress, and national and international economic and political institutions and that environmental change is not in itself a major driver of human violence. In the third phase, the Malthusian case appears to be stronger because human activities have reached a level where they have a truly global impact. Environmental optimists still insist that these problems can be overcome by human ingenuity and that the long-term trend towards less violence in human affairs is unlikely to be reversed by climate change. The stakes seem higher, but the structure of the debate remains largely the same.