When do states acquire nuclear weapons? This article introduces a strategic theory of nuclear proliferation that takes into account the security goals of all key actors: the potential proliferator, its adversaries, and, when present, its allies. In order to acquire nuclear weapons, we argue, a state must possess both the willingness and the opportunity to proliferate. Willingness requires the presence of a grave security threat that is not covered by a reliable ally. Opportunity requires high relative power vis-à-vis the state’s adversaries or the protection of an ally. While relatively weak states without a powerful ally lack the opportunity to go nuclear, those with a reliable ally that covers all their security goals lack the willingness to do so. Therefore, only powerful states or those protected by an ally that does not reliably cover some of their security goals will acquire the bomb. We evaluate our theory against all historical instances of nuclear development and trace its logic in the Soviet, Iraqi, Pakistani, South Korean, and West German cases. We conclude with implications for the study of proliferation and for U.S. non-proliferation policy.
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