In September 1862 the newly formed 20th Maine under the leadership of Adelbert Ames with Chamberlain as Lieutenant Colonel joined with the Third Brigade, First Division 5th Army Corps just before the battle of Antietam.
By May 1863 Chamberlain was colonel in command of the regiment just in time for the Battle of Gettysburg and by July he would have temporary command of the brigade whose regiments are as follows:
(June 6, 1864 - June 18, 1864)
Though Chamberlain would command these six regiments for less than two weeks he would still make a lasting impression on many of them.
These regiments made up the First Brigade of the First Division that was newly formed. Chamberlain having lost command of the Third Brigade to General Bartlett, a ranking officer who recently joined the Division, was given this new command that was equally impressive.
Barely able to walk Chamberlain returned to the Army Oct 11, 1864.
By this time the First Brigade had been reformed and contained only the following two regiments.
To make up for the loss Chamberlain was also assigned command of the Second Brigade.
✠ The cannoneer: recollections of service in the Army of the Potomac
The Third Brigade (see first column) by this time had lost the 44th New York, their enlistment being up, but had gained the following regiments.
Briefly commanding this brigade before and after the Battle of Petersburg, Chamberlain would not command it again until after the surrender.