People who read a long book deserve a short final chapter. Let me see if I can oblige. Deterrence is a fascinating subject because it is a core relationship among some of the major actors in international politics; for many analysts the nature of international politics is such that deterrence may be its most important interaction. Thus it is odd that it received so little study as a phenomenon in its own right until well into the twentieth century – that is a bit like wanting to study international economic relations without taking a close look at money. Then it began to get intense study. There seemed to be so much riding on it, in the nuclear age, that there was a terrible fear of the consequences if we ever got it wrong because we didn't know what we were doing.

My studies of deterrence have been moved by the following broad concerns. Deterrence came to be a central component of our security so it continues to be very important to understand it and practice it as best we can. But understanding it means facing up to the fact that it is inherently imperfect. It does not consistently work and we cannot manipulate it sufficiently to fix that and make it a completely reliable tool of statecraft. That means it must be approached with care and used as part of a larger tool kit.