Toward the end of the American Civil War, the Confederacy faced manpower shortages, and the Confederate Army, following practices the Union had already adopted, began to recruit soldiers from their prison ranks. They targeted foreign-born soldiers whom they thought might not have strong allegiances to the North. Key battalions included the Brooks Battalion, a unit composed entirely of Union soldiers who wished to join the Confederacy and were not formally recruited; Tucker's Regiment and the 8th Battalion Confederate Infantry recruited mainly among Irish, German, and French immigrants.

Though the scholarship on the Civil War is vast, Changing Sides represents the first entry to investigate Union POWs who fought for the Confederacy, filling a significant gap in the historiography of Civil War incarceration. To provide context, Patrick Garrow traces the history of the practice of recruiting troops from enemy POWs, noting the influence of the mostly immigrant San Patricios in the Mexican-American War. The author goes on to describe Confederate prisons, where conditions often provided ample incentive to change sides. Garrow's original archival research in an array of archival records, along with his archaeological excavation of the Confederate guard camp at Florence, South Carolina, in 2006, provide a wealth of data on the lives of these POWs, not only as they experienced imprisonment and being "galvanized" to the other side, but also what happened to them after the war was over.