Joining the Army of the Potomac on the eve of Antietam, Chamberlain participated in the battles of Shepherdstown Ford and Fredericksburg, where he was wounded, and won promotion to colonel and regimental command on 20 May 1863. At Gettysburg, on 2 July 1863, Chamberlain and the Twentieth Maine were directed to hold the extreme left flank of the Army of the Potomac, resting on Little Round Top. The colonel's able defense of this position played a significant role in turning back the Confederate assault, and in 1893 he was awarded the Medal of Honor for his personal heroism. Wounded during the battle, Chamberlain remained with his command until repeated attacks of malaria forced him to take medical leave from November 1863 to May 1864. Rejoining his regiment during the battle of Spotsylvania, Chamberlain was soon elevated to the command of a brigade, and he fought in the battles of the North Anna and Cold Harbor. On 18 June 1864 he was seriously wounded while leading an assault on the Petersburg fortifications, winning a battlefield promotion to brigadier general from Ulysses S. Grant. After recovering from his wounds, Chamberlain played a prominent role as a brigade commander in the Appomattox campaign, suffering yet another wound, and he commanded the Union forces designated to accept the formal surrender of the Army of Northern Virginia on 12 April 1865. His service during this final campaign earned him the brevet rank of major general of volunteers, a fitting coda to his distinguished military record.
Brooks D. Simpson, "Chamberlain, Joshua Lawrence," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00218.html.