In 1862-1863, well over 1,000 foreign-born men living in the United States argued that they had been illegally drafted into the Union military. Fearing a diplomatic row, the Lincoln Administration sought to clarify the rules of draft eligibility and its relation to citizenship. Secretary of State William Seward determined to include in the pool of potential draftees men who had filed their declaration of intent to become American citizens and men who had voted in any election in the United States. Seward’s policy, already in action in 1862, was partially codified in the Enrollment Act of March 3, 1863, and further enunciated in circulars from the Departments of State and War. The policy therefore broadened and redefined the nature of American citizenship. Highlighting the parallels to earlier forms of impressment, and breaking new ground with a quantitative analysis of the Department of State’s Case Files on Drafted Aliens, we argue that this previously unexplored history of forced military service for foreign-born soldiers during the American Civil War should be considered an example of impressment. With additional source material from foreign archives, we demonstrate the extent of Union impressment and its transnational character. We argue furthermore that concerns about forced service were widespread and had significant consequences for relations with European countries and for immigrant communities domestically. Official complaints about impressment represented a new kind of draft resistance, in which legal and political knowledge, as well as local and regional immigrant networks, were essential for securing freedom from military service.