Efforts to limit the proliferation of nuclear weapons and nuclear-weapons-related technology have increasingly involved economic, technological, and military forms of coercion implemented in an environment of low-level conflict. Coercive counterproliferation measures have included a range of actions, including targeted economic sanctions, industrial sabotage, cyber attacks, targeted killings, and military strikes. While the nonproliferation obligations of states are well-established under relevant treaties, state practice, and the international monitoring system of the International Atomic Energy Agency (IAEA), norms relating to the enforcement of those obligations are not clearly defined in legal instruments. This Article reviews the legality of prevention and enforcement measures through the institutional framework of the global nonproliferation regime, considering the tensions between that framework and a range of cross-cutting disciplines of international law, including the law of nonforcible intervention, state responsibility, and the law of force. The Article advocates the continuing development of consistent technical criteria to determine proliferation risk at the institutional level of the IAEA monitoring system, and the prioritization of that system in the enforcement of nonproliferation obligations. It addresses the key legal issues associated with the full range of counterproliferation prevention and enforcement options, providing a comprehensive framework to facilitate the refinement of legal norms guiding global counterproliferation efforts.