Fort Warren was built from 1833-1861 and was completed shortly after the beginning of the American Civil War. During the war, the island fort served as a prisoner-of-war camp for captured Confederate military and civilian officials, as well as Northern political prisoners.
James M. Mason and John Slidell, the Confederate diplomats involved in the Trent affair, were among those held at the fort. Military officers held at Fort Warren include Richard S. Ewell, Isaac R. Trimble, John Gregg, Adam "Stovepipe" Johnson, Simon Bolivar Buckner, Sr., and Lloyd Tilghman. High ranking civilians held at Fort Warren include Confederate Vice President Alexander H. Stephens, and Confederate Postmaster General John Henninger Reagan. The prison camp had a reputation for humane treatment of its detainees. When the camp commander's son entered the Union Army, he was given a letter from Confederate officers in the camp urging good care should he be captured. (He was later killed in combat).