This is the first comprehensive examination of the close relationship between leading groups of British socialists and American progressives in the early twentieth century. Employing new methods of conceptual and institutional analysis, and drawing on extensive original archival research, the book challenges traditional analyses of these movements. - ;In the first three decades of the twentieth century, two groups of radical political theorists - one British and one American - were bound together in a unique ideological relationship. Progressives, Pluralists, and the Problems of the State