Genocides have existed for as long as humans have recorded history. There are instances of the intentional destruction of an entire group of people in the Hebrew Bible, and the Romans destroyed Carthage in a manner that sought to make impossible the continued existence of Carthiginians. In the Middle Ages the papacy launched a crusade designed to annihilate physically every follower of the Albigensian heresy. Since the late fifteenth century instances of colonial genocides—in the Caribbean, North America, Australasia—have been entwined with the history of European expansion around the globe.

Some of these acts certainly had an ideological dimension. When the Israelites conquered Canaan and "devoted to destruction by the edge of the sword all in the city, both men and women, young and old" (Joshua 6:21–24), they were, in the Bible's recounting, inspired by God and his promises to them as his chosen people. The medieval church...