By historical standards, the early years of the twenty-first century have been remarkably peaceful. Only rarely are people killed by their own kind, and only very, very rarely are they killed by other animals, microorganisms excepted.  Nevertheless, even though the statistics should reassure, many people worry about lone killers, murderous gangs, and terrorist bands. At the same time, most people are vaguely aware that even in this relatively calm era, wars have made countless victims.   Yet mass violence against unarmed civilians has claimed three to four times as many lives in the past century as war: one hundred million at least, and possibly many more. These large-scale killings have required the efforts of hundreds of thousands of perpetrators.
