This article engages with the image of the League of Nations as an experiment in international law and the law of international institutions. This image populates international legal literature of the second half of the 20th century and of the 21st century. It corresponds to what is called here the “experiment narrative” about the League. Many of the claims made about international institutional law, collective security and international institutions in international legal discourses are informed by this specific narrative. Drawing on the “experiment narratives” about the League, this article shows that “experiment narratives” in international law constitute a common tool for international lawyers to uphold a progressive and linear global history and to organize their discourses.