At the close of the Civil War, more than Southern independence and the bones of the dead lay amid the smoking ruins of the Confederacy. Also lost was the memory of the prewar decades, when Southern politicians and pro-slavery ambitions shaped the foreign policy of the United States in order to protect slavery at home and advance its interests abroad. With “This Vast Southern Empire,” Matthew Karp, a professor of history at Princeton, recovers that forgotten history and presents it in fascinating and often surprising detail.

Although...