In the discussion of Doomsday machines in the book I take the conventional view that allout thermonuclear war would have destroyed both sides. Kahn argued (in On Thermonuclear War) that that view substantially overestimed the destructiveness of modern weapons. If he was right, nuclear retaliation might have made sense as a way of damaging the Soviet Union and so preventing an invasion of an injured but not destroyed U.S., making it in our interest ex post as well as ex ante.