The British Empire entered the twentieth century in a state of crisis, with many in the legal establishment fearing that the British constitution could no longer cope with the complexity of imperial institutions. At the same time, the military establishment feared the empire was becoming impossible to defend from multiplying threats. In this innovative study, Jesse Tumblin shows how Britain and its largest colonies, Canada, Australia, New Zealand, India, and South Africa, were swept up in a collective effort to secure the Empire in the early twentieth century. The hierarchy of colonial politics created powerful incentives for colonies to militarize before World War I, reshaping their constitutional and racial relationships toward a dream beyond colonial status. The colonial backstory of a century of war and violence shows how these dreams made 'security' the dominating feature of contemporary politics.