When do states acquire nuclear weapons? To address this question, a strategic theory of nuclear proliferation must take into account the security goals of all of the key actors: the potential proliferator, its adversaries, and, when present, its allies. To acquire nuclear weapons, a state must possess both the willingness and the opportunity to proliferate. Willingness requires the presence of a grave security threat against which no ally offers reliable protection. Opportunity requires that the state pursuing nuclear weapons possess high relative power vis-à-vis its adversaries or enjoy the protection of a powerful ally. Whereas a relatively weak state without a powerful ally lacks the opportunity to develop a nuclear capability, one with such an ally lacks the willingness to do so. Therefore, only powerful states or relatively weak states with allies that do not guarantee fulfillment of at least some of their key security goals will acquire the bomb. These claims are supported by the overall pattern of nuclear proliferation as well as detailed analyses of the Soviet, Iraqi, Pakistani, South Korean, and West German nuclear development cases.