When does a nuclear-armed state's provision of security guarantees to a militarily threatened ally inhibit the ally's nuclear weapons ambitions? Although the established security model of nuclear proliferation posits that clients will prefer to depend on a patron's extended nuclear deterrent, this proposition overlooks how military threats and doubts about the patron's intentions encourage clients to seek nuclear weapons of their own. To resolve this indeterminacy in the security model's explanation of nuclear restraint, it is necessary to account for the patron's use of alliance coercion, a strategy consisting of conditional threats of military abandonment to obtain compliance with the patron's demands. This strategy succeeds when the client is militarily dependent on the patron and when the patron provides assurances that threats of abandonment are conditional on the client's nuclear choices. Historical evidence from West Germany's nuclear decisionmaking provides a test of this logic. Contrary to the common belief among nonproliferation scholars, German leaders persistently doubted the credibility and durability of U.S. security guarantees and sought to acquire an independent nuclear deterrent. Rather than preferring to renounce nuclear armament, Germany was compelled to do so by U.S. threats of military abandonment, contradicting the established logic of the security model and affirming the logic of alliance coercion.

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