This study aims to examine the women’s peace movement in the context of the activism of transnational civil society that was influential in the formation of the idea of international relations before the First World War. The main argument of the study is that the women’s peace movement, which was institutionalized with the First World War, offers a different approach to international relations and peace than the idealist approach that prevailed in international relations after 1919. This argument was put forward by examining the establishment of the Women’s International League for Peace and Freedom (WILPF) and the International Women’s Congress held in Hague, 1915, and the International Women’s Congress in Zurich, 1919. At the end of the study, which aims to draw attention to the activism of transnational civil society in the early stages of international relations, WILPF’s approach to peace is discussed. Based on the experiences of women’s movements before the First World War, women’s activism that transcends nation-state borders towards peace provides a current understanding of the causes of war and the conditions of sustainable peace. The evaluation of the approach of the international relations put forward by the women’s movement together with its origins contributes to the feminist international relations literature with historical research.