A widely read and influential text in its own day, P.-J. Proudhon’s Du Principe federatif is now often overlooked by students of federalism. Yet the book’s theoretical and general chapters, in the first English translation, can claim to be considered a key text for the history of federalist thinking. Standing at the point of intersection between the anarchist and federalist traditions, they make a passionate case for federalism as the political order which gives the fullest possible expression to liberty – indeed, as the only political order in which liberty can be preserved: ‘The twentieth century will open the age of federations, or else humanity will undergo another purgatory of a thousand years.’ Proudhon’s federal principle is a radically decentralist one, which contrasts sharply with modern pictures of federalism at many points, what Proudhon calls a ‘federal’ system is what many, today, would regard as the dissolution of such a system. Although it thus stands apart from the mainstream of North American views of federalism, Proudhon’s book raises questions which are posed by any federal arrangement. In connecting the federalist ideal with such distinct ends as the dispersal of power, maximum participation, and the maintenance of cultural diversity, it builds significant political tensions into the concept of federalism itself.