Arbitrating Empire offers a new history of the emergence of the United States as a global power—one shaped as much by attempts to insulate the US justice system from international legal accountability as it was by efforts to project influence across the globe. Drawing on extensive archival research in the United States, Mexico, Panama, and the United Kingdom, the book traces how thousands of dispossessed residents of US-annexed territories petitioned international Claims Commissions between the 1870s and the 1930s to challenge endemic state violence under US governance. Through attention to the consequences of their unexpected claims, Allison Powers demonstrates how colonial subjects, refugees from slavery, and migrant workers transformed a series of tribunals designed to establish the legality of United States imperial interventions into sites through which to challenge the legitimacy of the US justice system itself. One of the first social histories of international law, the book argues that contests over meanings of sovereignty and state responsibility that would reshape the mid-twentieth century international order were waged not only at diplomatic conferences, but also in Arizona copper mines, Texas cotton fields, Samoan port cities, Cuban sugar plantations, and the locks and stops of the Panama Canal. Arbitrating Empire uncovers how ordinary people used international law to challenge the legitimacy of the US Empire and demonstrates why State Department attempts to erase their claims transformed international law in ways that continue to shield the US government from scrutiny to this day.