During most of human history, war was a basic instrument of statecraft, considered, for the most part, a lawful, honorable, ennobling, and even romantic pursuit. By contrast, peacemaking remained a marginal and indeed incongruous interstate activity. A war would end when the belligerents ended it. The experience of the twentieth century's two world wars has changed, at least, the official view. The introduction of ever more destructive weapons, the drastic escalation of civilian deaths, and the economic and environmental devastation that modern war brought combined to forge an international...