Deterrence as a concept is hardly new, dating back even to the Babylonian Code of Hammurabi in the seventeenth century BC, when virtually any serious crime was punishable by death. In fact, threats of retaliation and punishment are a feature of everyday life, from parents attempting to discipline their children to statesmen haggling over the details of a treaty. In the words of Thomas Schelling, “Nations, like people, are continually engaged in demonstrations of resolve, tests of nerve, and explorations for understandings and misunderstandings.” Given its ubiquitous presence, it is not surprising to discover that deterrence has attracted the attention of scholars and policymakers alike.

Yet, despite this interest, deterrence is still often misunderstood and remains an elusive concept. Leaders are dumbfounded when their threats are ignored, and equally struggle to discern whether the threats of their adversaries are genuine. Largely this is because deterrence is at root a psychological phenomenon, and it will never be possible to be certain of another person's state of mind. Even though some ambiguity will persist, however, it does not follow that the United States should categorically accept or reject the functioning of deterrence. Pursuing the former could invite the limited aggression and sanctuaries for terrorists mentioned in chapter 1. Adopting the latter is a recipe for global conflict and perhaps greater insecurity. A better understanding of the principles behind deterrence is the first step toward striking a middle ground between these extremes and crafting a sensible US policy against WMD proliferation.