We find that more closely related populations are more prone to engage in international conflict with each other. We provide an economic interpretation based on two connected mechanisms. First, more closely related groups share more similar preferences over rival goods and are thus more likely to fight over them. Second, rulers have stronger incentives to conquer populations more similar to their own, to minimize postconflict heterogeneity in preferences over government types and policies. We find support for these mechanisms using evidence on international conflicts over natural endowments and on territorial changes, including decolonization.