At first the atrocities went almost unnoticed. So many other momentous events were going on in the world—revolution in Russia, bloodshed in the Middle East, famine in Somalia. In the United States, people concentrated on presidential campaigns, economic recession, and the growing AIDS threat.

Yet the evidence was there from the beginning. Observers reported troop movements and war maps that warned of coming violence. "There could be 200,000 to 300,000 people slaughtered within a few months," said one worried official in late 1991.

Then there were the accounts of the war itself. Journalists reported that towns were being shelled, besieged, and burned. Hundreds of thousands of people had been driven from their homes. Neighbor had turned on neighbor. "They are shipping Muslim people through [town] in cattle cars," an anonymous caller told a Western journalist. "Last night there were...