In the first three decades of the 20th century, two groups of radical political theorists — one British and one American — were bound together in a unique ideological relationship. This book provides an examination of the intellectual dialogue that constituted that bond. Drawing on archival research, and employing methods of conceptual analysis, it examines the efforts of these two initially distinctive political movements to forge a single ideology capable of motivating far-reaching reform in both of their countries. In so doing, the book emphasizes the exceptional development of American progressivism and British socialism, arguing that the intellectual inspirations and political programmes of both movements were constantly shaped and reshaped by international ideological exchange. It analyses the complex political demands of these movements and enables the works of their leading protagonists, including G. D. H. Cole, Herbert Croly, Harold Laski, and Walter Lippmann, to emerge as significant contributions to modern political thought.