In September 1863, two divisions of the XI Corps, those of Steinwehr and MG Carl Schurz, were transferred to the Western Theater to help relieve the besieged Union army in Chattanooga, becoming part of the Army of the Cumberland. They served under Maj. Gen. Joseph Hooker in the Battle of Wauhatchie, where the brigade of Col Orland Smith from Steinwehr's division distinguished itself. Buschbeck's brigade was engaged alongside Maj. Gen. William T. Sherman's command at the Third Battle of Chattanooga. After that battle the XI Corps was combined with the equally depleted XII Corps to form the new XX Corps. That corps fought under Maj. Gen. William T. Sherman in the Atlanta Campaign and March to the Sea, but Steinwehr was essentially reorganized out of his job and he commanded no more combat units during the war. He resigned his commission on July 3, 1865.