Crimes against humanity and genocide may be seen as realities distinct from more normal human events, thus requiring a distinctive historiography. Cruelty carried to the point of genocide is abnormal in two ways: It violates moral norms that are central to many ethical and religious traditions, and the vast majority of people do not engage in such actions, or else feel guilt or discomfort if they do. Although continuity undoubtedly exists between normal, everyday human wrong-doing and genocide and crimes against humanity, these acts ought to occasion a special sense of revulsion, for by the deliberate intentions and actions of human beings, they lay waste to entire human worlds and leave ruin in their wake. The same can also be said of systematic and deliberate violations of human rights.

Large-scale atrocity raises peculiar difficulties for historians. First, it tends to wipe out those who know atrocity most intimately: the...