This article reappraises the debate about war prevention in the Bryce Group, the first study circle in Britain to devise a plan for the League of Nations. While scholars have tended to associate pro–league of nations activists with idealism, more focused accounts of the group have mostly depicted its postwar plan as a product of realistic thinking. Drawing on the underused manuscripts of the intellectual founders of the League of Nations, this study first reveals that their early thinking defies simple categorization. Not only was their war prevention plan realistic about the role of armed force but it also depended critically on idealistic expectations about the moral force of public opinion. This article shows that realistic and idealistic views could rarely be separated, and both developed the group's plan for peace, which incorporated the collective use of force as a crucial element of the postwar order. A mixture of the two views, however, hardly ensured consistencies and a balance between them. The paradox of collective security discussed by the group in 1914–15—that peace at least in part rested on the threat of force—was unresolved by the foundation of the League, and remains intact to this day.