This book explores the development, character, and legacy of the ideology of liberal internationalism in late nineteenth and early twentieth-century Britain. Liberal internationalism provided a powerful way of theorising and imagining international relations, and it dominated well-informed political discourse at a time when Britain was the most powerful country in the world. Its proponents focused on securing progress, generating order and enacting justice in international affairs. Liberal internationalism united a diverse group of intellectuals and public figures, and it left a lasting legacy in the twentieth century. This book elucidates the roots, trajectory, and diversity of liberal internationalism, focusing in particular on three intellectual languages – international law, philosophy and history – through which it was promulgated. Finally, it traces the impact of these ideas across the defining moment of the First World War. The liberal internationalist vision of the late-nineteenth century remained popular well into the twentieth century and forms an important backdrop to the development of the academic study of International Relations in Britain.