 At the outbreak of the Civil War, he was commissioned brigadier general and steadily rose from brigade commander, to division commander, to commander of the I Corps, which he led during the Battle of Antietam.Following that battle, he was placed in command of the V Corps and then the Center Grand Division of the Army of the Potomac, consisting of the III and V Corps.On January 26, 1863, he was promoted to command of the Army of the Potomac.