Feminists have utilized manifestos and utopias in order to make important, often revolutionary, contributions to international law. However, these engagements have not been reflected in the substance of international law. The sources of international law – specifically customary international law – rely on a narrow understanding of historical knowledge. This article centres the 1924 manifesto and the ‘New International Order’ created by the Women’s International League for Peace and Freedom, as tools to assess the exclusion of the under-utilized history of feminist peace work from the sources of international law. This allows for a reflection on customary international law’s weaknesses and reaffirms the importance of feminist approaches to international law.