The ideal of the United Nations was first put forward by Immanuel Kant in his 1795 essay Perpetual Peace. Kant, in the tradition of Locke and Rousseau is a liberal who believes that relations between individuals can either be based upon law and consent or upon force and violence. One way that such the ideal of world peace could be achieved would be through the creation of a single world state, of which every human being was a citizen. Such an ideal was advocated by a number of eighteenth century liberals. Kant, however, rejects this ideal and instead argues that the universal rule of law can be achieved through the establishment a federation of independent states. I examine the relevance of Kant’s arguments today, focusing on two questions: Firstly, as advocates of the rule of law, why advocate a federation of independent nations rather than a single world state. Secondly, is this ideal realizable? Is Kant right to think that republics are natural and are likely to live peacefully with one another? Kant’s arguments on this issue have been taken up again in recent decades by defenders of the theory of the “democratic peace”, the theory that democracies are more likely to live at peace with one another.