In a familiar pattern, federal judges ultimately embraced their role as the architects of American sovereignty on the water. As the Monroe administration redoubled its prosecutions of South American privateers, Congress left it to judges to define the legitimate boundaries of maritime violence. The Supreme Court responded by casting doubt on the claims to sovereignty advanced by revolutionary polities, and declaring that privateers were merely pirates, and therefore subject to punishment by all – including the United States. This judicial assertion of legal authority to police the waters of the revolutionary Atlantic was transformative. It helped secure approval of a treaty with Spain that paved the way for decades of territorial expansion in North America, and it presaged increasingly expansive American claims to hemispherical preeminence. Even when federal judges denied their own power to discipline a different category of “pirates” – those who engaged in the slave trade – they did so to uphold sovereign rights that Americans had been asserting since independence. If a nineteenth century American empire was ultimately realized on land, some of its first stirrings were at sea.