What is the purpose, if any, of the nuclear bomb, that brooding presence that has shadowed all human life for sixty-five years? The question has haunted the nuclear age. It may be that no satisfactory answer has ever been given. Nuclear strategic thinking, in particular, has disappointed. Many of its pioneers have wound up in a state of something like despair regarding their art. Now a new moment, full of fresh promise but also with novel perils, has arrived in the nuclear story, and all the old questions have to be asked again.