In 1862, Lincoln broke with longstanding US policy by agreeing with Britain to establish international courts to suppress the Atlantic slave trade. Historians have previously neglected these courts because the illicit trade’s decline in the 1860s resulted in empty dockets. This article, however, shows how the two American commissioners on the court at Freetown took up a broader intervention in Africa based in part on British imperial practices that the commissioners viewed as consistent with federal policies back home. Most notably, they proposed a treaty system with African nations that would exchange American protection, trade, and “civilizing” reforms for commitments to end slavery and its trade. This lost international history of the Civil War thus extends the history of American antislavery expansionism in West Africa into the Civil War period and captures a revealing vision of American expansion during the war beyond the consolidation of a continental empire.