This article questions two interrelated myths pertaining to the interwar internationalism of the British Labour Party and the theories of so‐called idealists in the academic discipline of International Relations (IR). In IR, interwar “idealists” are (in)famous for a detached and utopian approach to international politics. Conventional historiographical verdicts on the international policy of the Labour Party in the interwar period suggest that the party was the practical mirror of this naïve international outlook. In fact, the two themes are connected, most notably through Labour's Advisory Committee on International Questions. This article brings the study of Labour's internationalism and the international theories of purported idealists together by focusing on debates on the League of Nations and the use of force. The analysis reveals that conventional historiographical narratives are inadequate and too simplistic for grasping the diversity of Labour's internationalism and interwar progressivist ideas about international politics in general.