It was Saint Augustine who first proposed that it might be acceptable to preemptively attack a robber before he sets upon his mark. It is fair game to attack “an assassin lying in ambush,” Augustine noted in his treatise On Free Will, “even before the crime has been committed.” Throughout the subsequent history of Western moral philosophy, the supposition that the pursuit of one evil could forestall a greater one has had a long and checkered legacy. The lesser-evil rationale for otherwise culpable conduct, moreover, continues to raise ethical questions. In Augustine’s scenario, how