In December 1863, Foster was sent to Tennessee to assume command of the Department of the Ohio and its corresponding Army of the Ohio. He was in command for only a short time before he was badly injured in a fall from his horse. Upon his recovery, he took command of the Department of the South and aided in forcing the surrender of Savannah, Georgia. He was making preparations for the surrender of Charleston, but his wounds forced him to relinquish command to Maj. Gen. Quincy A. Gilmore.