This paper studies the role of leaders in the social movement against slavery that culminated in the US Civil War. Our analysis is organized around a natural experiment: leaders of the failed German revolution of 1848–1849 were expelled to the United States and became antislavery campaigners who helped mobilize Union Army volunteers. Towns where Forty-Eighters settled show two-thirds higher Union Army enlistments. Their influence worked through local newspapers and social clubs. Going beyond enlistment decisions, Forty-Eighters reduced their companies' desertion rate during the war. In the long run, Forty-Eighter towns were more likely to form a local chapter of the NAACP.