Where does international law (IL) draw its authority from a still weakly institutionalized international scene deprived of the warrants of a state? To address this classical debate, the article draws from a case study on the social and professional structure of the “international legal community” as it emerged during the 1920s as part of the rise of multilateralism and international organizations. It focuses on the “situation of the international lawyers” of the time, starting with the multiple and often antagonistic roles they play (as legal advisers, scholars, judges, diplomats, politicians, etc.) and the variety of interests and causes they defend (states, international organizations, professional interests, etc.) in international politics. It argues this heteronomy of international lawyers helps understand the autonomization of international law. Far from being opposed to one another it has often been assumed—realism and idealism, national loyalty and international loyalty, political logic and learned logic—actually gain when analyzed as various modes of affirming a single cause—that of an international rule of law. This attention given to the “situation of international lawyers” and to the way they manage their various allegiances also accounts for the particular vision of the “International” and of “Law and Politics” relationships that are encapsulated in this emerging international corpus juris.