This article examines the issue of coercively preventing states from acquiring and possessing nuclear weapons. In questioning whether such coercion is morally legitimate, I argue that Immanuel Kant’s (1724-1804) political theory contains important resources compared with three rival perspectives: Realpolitik, the Just War Tradition, and Deontological Pacifism. I also argue that coercive anti-proliferation measures are conditionally legitimated by three distinctive Kantian concepts: First, his concept of International Justice allows for coercion against genuinely aggressive states engaged in nuclear aspiration. Second, given the imperfections of international justice institutions, his concept of a State of Peoples – an authorized global governance body – seems to provide a better guarantee of just forms of coercive nuclear anti-proliferation. Third, supplementing the first two concepts, Kant discusses a Cosmopolitan Right to share the earth’s surface. This concept justifies coercive anti-proliferation when a people’s right to existence as citizens of the earth is threatened by nuclear weapons.