Between 1917 and 1922, Leavenworth Federal Penitentiary was occupied by a unique mix of soldiers, war dissenters, radical labor organizers, foreign-born radicals, Black militants, and Mexican revolutionaries. Prisoners included figures like Ricardo Flores Magón, one of the central theorists and agitators of the Mexican Revolution. The article observes how incarcerated revolutionaries and working-class soldiers coordinated night schools, produced their own newspaper, led May Day marches, initiated strikes, and continued agitating and educating one another in the prison. Drawing from prison records, inmate book collections, correspondence, and federal surveillance records, archives collected from across the United States, Mexico City, and Amsterdam, the article explores the unanticipated alliances and political struggles that arose from this unique convergence space. It subsequently describes how a unique form of radical internationalism was theorized within and against an emergent security infrastructure.