It is commonly argued that Realist international relations theory is thoroughly statist and provides no hope or scope for the transformation of politics on a global scale. In Politics Among Nations, however, Hans Morgenthau devotes an entire chapter to the subject of a world state and clearly presents it as the only possible path toward attaining world peace. Here, Morgenthau suggests that a world community needs to be forged before a world state could exist as anything more than ‘a totalitarian monster resting on feet of clay’, an argument that seems more at home in Kantian cosmopolitanism or English School theory than it does in mainstream representations of power politics realism. This paper aims to investigate Morgenthau’s argument in order to understand the ethical claims he advances, particularly as they relate to the problem of sovereignty. It will be argued that, while having some cosmopolitan sympathies, Morgenthau remains attached to a decisionist understanding of state sovereignty that remains relevant even in a world state. It is this attachment to a de facto account of state sovereignty that differentiates Morgenthau’s version of the world state from those presented by his contemporary critics. Morgenthau’s world state, in other words, maintains the unsettling centrality of power at the heart of an idealised vision of world peace.