Henry Halleck
Prior to 1903, the senior military officer in the army was the Commanding General of the United States Army, who reported to the Secretary of War. From 1864 to 1865, Major General Henry Halleck (who had previously been Commanding General) served as "Chief of Staff of the Army" under the Commanding General, Lieutenant General Ulysses S. Grant, thus serving in a different office and not as the senior officer in the army.