The vast majority of the world's countries are experiencing a demographic revolution: dramatic, sustained, and likely irreversible population aging. States' median ages are steadily increasing as the number of people ages 65 and older skyrockets. Analysts and policymakers frequently decry population aging's domestic costs, especially likely slowing economic growth and massive new public expenditures for elderly welfare. But aging has a major yet largely unrecognised international benefit: it significantly reduces the likelihood of international war. Although wars continue to rage in parts of the world, almost none involve aged countries. This book provides a comprehensive and groundbreaking argument why population aging will be a powerful force for peace.