William F. Fox, Lt. Col. U. S. V., Regimental Losses in the American Civil War, 1861-1865: A Treatise on the extent and nature of the mortuary losses in the Union regiments, with full and exhaustive statistics compiled from the official records on file in the state military bureaus and at Washington

The organization of the Twelfth Corps may be considered as dating from the General Order of March 13, 1862, under which the corps formation of the Army of the Potomac was first created. By that order, five different corps were constituted, one of which, composed of the divisions of Williams and Shields, and commanded by General Banks, was designated as the Fifth. These divisions were then operating in the Shenandoah Valley. On the 26th of June, the President ordered that “the troops of the Shenandoah Department, now under General Banks, shall constitute the Second Army Corps” of the Army of Virginia. On September 12th, General Order 129, it was ordered that its designation be changed to that of the Twelfth Corps, and that General Joseph K. Mansfield be placed in command.

In the meantime the corps had done considerable hard fighting under its former title. Shields' Division won a brilliant victory over Stonewall Jackson at Kernstown, Va., on the 23d of March, and Williams' Division fought well at Winchester, May 25th, 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 slows that the two divisions did there some of the best fighting of the War. In that battle the divisions were commanded by Generals Williams and Augur; loss, 302 killed, 1,320 wounded, and 594 missing; total, 2,216, out of less than 6,000 engaged. This loss fell oil four brigades, Crawford's Brigade losing 867 men out of 1679, reported by Crawford as “present in engagement.” At Manassas the corps was held in reserve.

It participated in the Antietam campaign under its proper designation, as the Twelfth Corps, with the veteran Mansfield in command. Its division and brigade organization was the same as at Cedar Mountain; General George S. Greene had succeeded General Augur in the command of the Second 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 oily. The corps now numbered 12,300 present for duty, including the non-combatants; it contained 22 regiments of infantry, and 3 batteries of light artillery. It was the smallest corps in the Army.

It was not engaged at South Mountain, although it marched thither in plain view of the battle which was raging on the mountain's side, ahead of its dusty columns. At 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, 1,38（6 wounded, and 85 missing; total, 1,744), out of about 8,000 present in action.

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

The brunt of the battle of Chancellorsville fell on the Third and Twelfth Corps; and yet amid all the rout and confusion of that disastrous battle the regiments of the Twelfth Corps moved steadily with unbroken fronts, retiring at the close of the battle without the loss of a color, while the corps' artillery, after having been engaged in the close fighting at the Chancellor House, withdrew in good order, taking every gun with them. In this campaign Slocum's troops were the first to cross the Rapidan, and the last to re-cross the Rappahannock. The corps at this time contained 30 regiments of infantry, with 5 batteries of light artillery, numbering in all 12,929 present for duty, Its losses at Chancellorsville amounted to 260