Chapter 1 examines the social and intellectual backgrounds of the pro-league of nations movement. Although previous research has focused on the movement’s activity predominantly during the inter-war period, the post-war plan emerged from an older European intellectual tradition. This chapter, therefore, contextualises the pro-league movement into this rich legacy by exploring two broader contexts: the immediate backdrop to the evolution of the movement and the history of ideas about war and peace. In the pre-1914 period, the future pro-league activists already had networks of influence that became the basis of a pro-league movement. While they drew upon a European intellectual legacy, the problems they faced differed from those of their predecessors – the breakdown of the Concert of Europe and the rise of nationalism. These problems led the pro-leaguers to not only develop fresh perspectives on the causes of war, but also conclude that a new world order should be established.