My book manuscript investigates when great power nonproliferation diplomacy succeeds or fails. I argue that this outcome depends on two variables: first, the nuclear aspirant’s security environment, and, second, the policy tools employed by the great power. When a state faces an offensive military threat from a conventionally superior adversary, it has a powerful incentive for proliferation. To restrain that state’s ambitions, the great power will need to threaten military or economic costs for nuclear defiance, and, in parallel, offer security guarantees following nuclear restraint. Without security guarantees, a militarily threatened state will be willing to suffer the costs of coercion to satisfy its proliferation imperative. But without coercion, states in precarious military environments will seek nuclear weapons even if they receive security guarantees, due to the catastrophic costs associated with military abandonment. In short, without both security guarantees and coercive leverage, nonproliferation diplomacy with militarily threatened states will fail.

By contrast, when a state does not face military threats from conventionally superior adversaries, it enjoys a more permissive security environment. Under this circumstance, a great power can achieve diplomatic success using either coercive pressure or security guarantees. Because secure states pursue nuclear weapons as a hedge, their imperative for proliferation is weak. As such, they will be unwilling to suffer painful costs to acquire nuclear weapons, and the great power’s use of coercive pressure will be enough to deter their ambitions. Similarly, if such states receive security guarantees, they will opt to rely on a patron’s extended deterrent rather than pursue an independent arsenal, since the latter could trigger a costly arms race with neighboring states. Without security guarantees or coercion, however, even aspirants in permissive security environments will continue down the nuclear path, as they will lack any incentive to abandon it.

My theory challenges two important arguments in the academic and policy debates over nonproliferation. First, by identifying coercive leverage as a key driver of nuclear reversals, my theory contests the idea that nuclear decisions are rooted primarily in unit-level variables such as regime type, economic openness, or elite psychology, and it contributes to a growing literature that highlights the critical role of U.S. coercive diplomacy in constructing and preserving the global nonproliferation regime. At the same time, my book critiques the claim that coercive pressure alone will necessarily deter states from acquiring nuclear weapons. Rather, for militarily threatened states, both coercive leverage and security guarantees will be necessary to produce nuclear restraint—in the absence of one or the other, incentives for proliferation will persist.

To test my argument, my manuscript identifies every nuclear aspirant that was subjected to U.S. nonproliferation demands. It then uses two complimentary methods of inquiry: analysis of cross-case variation and historical process tracing. In the first step, I code my theory’s expectations for each state in the universe of cases and assess the proportion of outcomes that my theory correctly predicts. Then, to verify that both the timing of these outcomes and the decision-making process that led to their occurrence are consistent with my theory’s logic, I employ detailed process tracing on a series of representative case studies: West Germany, Libya, South Africa, and Pakistan. To broaden my argument’s scope, I am currently completing two additional case studies: Japan and North Korea, and my conclusion addresses U.S. policy toward Iran.