The twentieth century was the bloodiest in human history. Millions died in battle, of course, but even more-estimates range from 60 million to 150 million-were innocent victims of genocide and mass slaughter. In trying to make sense of such violence, scholars have tended to look within societies: at collective psychology, ethnic and racial hatred, and the character of government. In this astute and provocative study, Valentino argues instead that leaders, not societies, are to blame. In most cases, he finds that powerful leaders use mass killing to advance their own interests or indulge their own hatreds, rather than to carry out the desires of their constituencies. This "strategic" view emerges from a review of communist mass killing in the Soviet Union, China, and Cambodia; ethnic killing in Turkish Armenia, Nazi Germany, and Rwanda; and counter-guerrilla killing in Guatemala and Afghanistan. Indifference and passivity were pervasive among the public, but it was the leaders who saw these bloody episodes as a solution to a problem. Valentino cleverly notes that if mass killing is not deeply rooted in society but a tactic of state power, the rest of the world has fewer excuses for inaction.