This is the first book-length study of the ideological foundations of British imperialism in the early twentieth century. By focussing on the heretofore understudied concept of imperial citizenship, the book illustrates how the political, cultural, and intellectual underpinnings of empire were constructed and challenged by forces in both Britain and the 'Britains Overseas, ' the settlement colonies of Canada, South Africa, New Zealand and Australia. Debates about imperial citizenship reveal how Britons conceived of the empire: was it an extension of the nation-state, a collection of separate and distinct communities, or a type of 'world-state'? These debates were also about the place of empire in British society, its importance to the national identity, and the degree to which imperial subjects were or were not seen as "fellow Britons." This public discourse was at its most fervent from the Anglo-Boer War (1899-1902) to the early 1920s, when Britain emerged victorious, shocked, and exhausted from the Great War. Drawing on the thinking of imperial activists, publicists, ideologues, and travellers such as Lionel Curtis, John Buchan, Arnold White, Richard Jebb, and Thomas Sedgwick, the book is a comparative history of how the idea of imperial citizenship took hold in early 20th c. Britain, and how it helped foster the articulation of a broader British World. It also reveals how imperial citizenship as a form of imperial identity was challenged by voices in both Britain and the empire, and how it influenced later imperial developments such as the immigration to Britain of 'imperial citizens' from the colonies after the Second World War. A work of political, intellectual, and cultural history, the book re-incorporate the histories of the settlement colonies into imperial history, and suggests the importance of comparative history in understanding the imperial endeavour. It should be of interest to students of imperialism, British political and intellectual history, and of the various former dominions, as well as readers broadly interested in Britain's imperial past.