Chapter 2 focuses on the debate about war prevention in the Bryce Group, the first pro-league circle in Britain. While scholars have tended to associate the pro-league activists with utopianism, some focused studies of the group have mostly depicted its post-war plan as a product of realistic thinking. This chapter reveals that their early thinking defies simple categorisation. 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. Realistic and idealistic views could rarely be separated, and both of them developed the group’s plan for peace which incorporated the collective use of force as a crucial element of the post-war order. Although the group attempted to maintain a balance between the two views, the result was inconsistencies and contradictions, which remained in the war prevention system of the League of Nations.