Examining how, and to what effect, the phrase “truly international” became central to the rhetoric and organization of the American-hosted 1950 International Congress of Mathematicians, this essay traces the negotiation of a “truly international” discipline from mathematicians’ first international congresses around the turn of the century across two world wars and their divisive interlude. Two failed attempts to host international congresses of mathematicians in the United States, for 1924 and 1940, defined the stakes for those who became the principal organizers (...) for 1950. Combining American organizational records with contexts and sources that extend across and beyond traditional mathematical centers in Europe and North America, the essay shows how a small cohort of American mathematicians marshaled an emphatic but ambiguous “international” rhetoric to guide policies and command cooperation and support while responding to persistent challenges. Their adaptations and compromises left a lasting mark on the terms and achievements of international inclusion, cooperation, and hegemony in mathematics. (shrink)