Many of the wars fought in the second half of the twentieth century—in Israel, Turkey, Rwanda, Sudan, Angola, Chechnya, Bosnia, and Kosovo, for example—are ethnic conflicts involving groups of people who had, in most cases, lived peacefully side- by-side for years before their country erupted into violence. Throughout these periods of stability, however, tensions may be growing until some incident sparks civil war. Once ignited, ethnic hostilities may not cease until one side has “cleansed”— killed or removed—the other side from the shared territory.

Outside nations are usually reluctant to become involved in ethnic wars, rationalizing that it is an “internal” problem and not the concern of others. But occasionally, public outcry over the potential genocide of an entire population compels foreign governments to step in and try and make peace...