At the outset of the war, Hooker became a colonel of the state militia. After the Battle of First Manassas, he received his commission as a brigadier general in August 1861 in command of brigade in the Division of the Potomac. In October 1861, he advanced to command a division of Army of the Potomac. He served at the siege operations at Yorktown, and distinguished himself at the Battle of Williamsburg. In September, he received his promotion to corps commander. He served at the Battle of Antietam, and afterward witnessed the succession of General Ambrose Burnside to the command of the Army of the Potomac. After the reorganization of the army, he was put in command of the Center Grand Division of the Army of the Potomac. After the Battle of Fredericksburg, he assumed the role of commander of the army from General Burnside. He reorganized and revitalized the army, but he failed to win a victory in May 1863 at the Battle of Chancellorsville. He resigned his command in June 1863. With the Union defeat at the Battle of Chickamauga, he was given charge of the army of the Potomac's 1lth and 12th Corps. In November 1863, he distinguished himself in the capture of Lookout Mountain. In the spring of 1864, the two corps were merged into the new 20th Corps, which he led through the Atlanta Campaign. Afterward, he asked to be relieved. He finished the war in the Northern department of Michigan, Ohio, Indiana, and Illinois.