When the United States emerged as a world power in the years before the Civil War, the men who presided over the nation’s triumphant territorial and economic expansion were largely southern slaveholders. As presidents, cabinet officers, and diplomats, slaveholding leaders controlled the main levers of foreign policy inside an increasingly powerful American state. For proslavery leaders like John C. Calhoun and Jefferson Davis, the nineteenth-century world was torn between two hostile forces: a rising movement against bondage, and an Atlantic plantation system that was larger and more productive than ever before. In this great struggle, southern statesmen saw the United States as slavery’s most powerful champion. Overcoming traditional qualms about a strong central government, slaveholding leaders harnessed the power of the state to defend slavery abroad. During the antebellum years, they worked energetically to modernize the U.S. military, while steering American diplomacy to protect slavery in Brazil, Cuba, and the Republic of Texas.According to my guest, these leaders were nationalists, not separatists. Their “vast southern empire” was not an independent South but the entire United States, and only the election of Abraham Lincoln broke their grip on national power. Fortified by years at the helm of U.S. foreign affairs, slaveholding elites formed their own Confederacy—not only as a desperate effort to preserve their property but as a confident bid to shape the future of the Atlantic world.To unpack this fascinating relationship between slavery and American foreign policy is Matthew Karp.Matthew Karp is Assistant Professor of History at Princeton University and is a historian of the U.S. Civil War era and its relationship to the nineteenth-century world. He received his Ph.D. in History from the University of Pennsylvania in 2011 and joined the Princeton University faculty in 2013. His first book, This Vast Southern Empire: Slaveholders at the Helm of American Foreign Policy explores the ways that slavery shaped U.S. foreign relations before the Civil War. This Vast Southern Empire received the John H. Dunning Prize from the American Historical Association, the James Broussard Prize from the Society for Historians of the Early American Republic, and the Stuart L. Bernath Prize from the Society for Historians of American Foreign Relations. Karp is now at work on a book about the emergence of anti-slavery mass politics in the United States and in particular the radical vision of the Republican Party in the 1850s. You can him follow on Twitter @karpmj.