XII Corps (Union Army) - The Valley and Antietam

The Valley and Antietam

The corps had done considerable hard fighting under its former title. Shields's Division won a victory over Thomas J. "Stonewall" Jackson at Kernstown on March 23, and Williams' Division fought well at Winchester, May 25, while on Banks' retreat. The Battle of Cedar Mountain was also fought by this corps, alone and unassisted; and, although defeated by the overwhelming force of the enemy, the record shows that the two divisions did some of the best fighting of the war there. In that battle the divisions were commanded by Generals Williams and Christopher C. Augur; loss, 302 killed, 1,320 wounded, and 594 missing; total, 2,216, out of less than 6,000 engaged. This loss fell on four brigades, Samuel W. Crawford's Brigade losing 867 men out of 1679, reported by Crawford as "present in engagement". At the Second Battle of Bull Run the corps was held in reserve.

The corps participated in the Maryland Campaign as the XII Corps, with the veteran Mansfield in command. Its division and brigade organization were the same as at Cedar Mountain; Brig. Gen. George S. Greene had succeeded Augur in the command of the 2nd Division. Its depleted columns had been strengthened by the accession of five new regiments of volunteers fresh from the North, three of which were composed of Pennsylvanians enlisted for nine months only. The corps now numbered 12,300 present for duty, including the non-combatants; it contained 22 regiments of infantry and three batteries of light artillery. It was the smallest corps in the army.

It was not engaged at South Mountain, although it marched in plain view of the battle that was raging on the mountain's side, ahead of its dusty columns. At the Battle of Antietam, it entered the fight early in the morning, and carried a position near, and in front of, the Dunker Church. General Mansfield fell, mortally wounded, while deploying his columns, and the command of the corps during the battle devolved on General Williams. The two divisions lost in this battle, 275 killed, 1,386 wounded, and 85 missing; total, 1,746, out of about 8,000 present in action.

The vacancy caused by the death of General Mansfield was filled by the appointment of Maj. Gen. Henry W. Slocum, a division commander of the VI Corps, who had already achieved a brilliant reputation by his services on in the Peninsula Campaign, and at the successful storming of Crampton's Gap. The XII Corps remained in the vicinity of Harpers Ferry until December, when it moved into Virginia, and made its winter quarters at Stafford Court House.