After Second Bull Run, McLean and his regiments were posted in defense of Washington. Here, the Army of Virginia was discontinued (Pope would be sent to Minnesota for his failures), and the I Corps became Maj. Gen. Franz Sigel's XI Corps of the Army of the Potomac under Maj. Gen. George B. McClellan. As part of the XI Corps, the brigade stayed posted in Washington during the Battle of Antietam in September, and they were not called on to move again until December. At this point, McClellan had been replaced by Maj. Gen. Ambrose Burnside, who called the XI Corps to Fredericksburg, but kept them in reserve during the Battle of Fredericksburg in mid-December. Afterwards, McLean was put in charge of protecting a small town on the Northern Neck.