XII Corps ( Union Army ) The XII Corps ( Twelfth Army Corps ) was a corps of the Union Army during the American Civil War . The corps was formed by U . S . War Department General Order of March 13 , 1862 , under which the corps organization 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 Alpheus S . Williams and James Shields and commanded by Maj . Gen . Nathaniel P . Banks , was designated as the V Corps . These divisions were then operating in the Shenandoah Valley . On June 26 , President Abraham Lincoln 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 12 , General Order 129 , it was ordered that its designation be changed to that of the XII Corps , and that Maj . Gen . Joseph K . Mansfield be placed in command . The XII Corps was small — only two divisions instead of the customary three — but was composed of excellent material . Among its regiments were the 2nd Massachusetts , 7th Ohio , 5th Connecticut , 13th New Jersey , 107th New York , 28th Pennsylvania , 46th Pennsylvania , 3rd Wisconsin , and others equally famous as crack regiments ; all of them with household names in the communities from which they were recruited . 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 . Chancellorsville and Gettysburg . The brunt of the Battle of Chancellorsville fell on the XI and XII Corps ; and yet amid all the rout and confusion of that disastrous battle the regiments of the XII 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 River , and the last to re - cross the Rappahannock River . The corps at this time contained 30 regiments of infantry , with five batteries of light artillery , numbering in all 19 , 929 present for duty . Its losses at Chancellorsville amounted to 260 killed , 1 , 436 wounded , and 1 , 118 missing ; total , 2 , 814 . The hardest fighting and heaviest losses fell on the brigades of Brig . Gen . Thomas H . Ruger and Col . Charles Candy . The divisions were commanded by Generals Williams and John W . Geary . At the Battle of Gettysburg , the XII Corps distinguished itself by its gallant defense of Culp ' s Hill on July 2 – 3 , 1863 . General Slocum was in command of the right wing at Gettysburg , which left Alpheus S . Williams , of the 1st Division , in command of the corps ; Thomas H . Ruger of the 3rd Brigade , 1st Division , took Williams ' s place as commander of the division ; Geary commanded the 2nd Division . On the afternoon of July 2 the corps was ordered by army commander George G . Meade to disengage from Culp ' s Hill and reinforce the Union line on its extreme left flank , near Little Round Top . Slocum persuaded Meade to leave one brigade behind to hold the critical position : Greene ' s Brigade , of Geary ' s Division . While occupying this position on Culp ' s Hill , with an excessively long line to defend and no other troops in support , Greene was attacked by Edward " Allegheny " Johnson ' s Division , but the attack was successfully repulsed by his brigade . Greene , a civil engineer , had insisted that his men prepare impressive defensive works on the hill . Still , some of Johnson ' s troops effected , without opposition , a lodgment in the vacated breastworks of the XII Corps , and upon the return of those troops a desperate battle ensued to drive the Confederates out . After a long , hard fight the corps succeeded in re - occupying its works . On no part of the field did the Confederate dead lie thicker than in front of the XII Corps position . Johnson ' s Division , containing 22 regiments , lost in this particular action , 229 killed , 1 , 269 wounded , and 375 missing ; total , 1 , 873 . To this must be added the losses suffered in the 14 regiments from the brigades of William Smith , Junius Daniel , and Edward A . O ' Neal , which were sent to Johnson ' s support . The XII Corps , containing 28 regiments , lost 204 killed , 810 wounded , and 67 missing ; total , 1 , 081 . Tennessee . At the conclusion of the Gettysburg Campaign , the Army of the Potomac pursued Robert E . Lee into Virginia , the XII Corps joining in the pursuit , and pushing forward until it reached the Rappahannock . While encamped there , on September 23 , 1863 , the XI and XII corps were detached from the army and ordered to Tennessee as a reinforcement for William Rosecrans , besieged in Chattanooga . The two corps were placed under the command of Maj . Gen . Joseph Hooker . Arriving in Tennessee , Geary ' s Division moved to the front , while Williams ' s Division was stationed along the railroad from Murfreesboro to Bridgeport . Geary pushed on in order to effect a junction with the beleaguered army at Chattanooga . On the night of October 27 , his division bivouacked in Lookout Valley , in an advanced and isolated position , where he was attacked at midnight by a part of James Longstreet ' s command . But Geary had taken proper precautions against surprise , and Longstreet was repulsed , Geary receiving in this affair a prompt and gallant support from part of the XI Corps . General George H . Thomas , commanding the Army of the Cumberland , stated in his official report that " the repulse by Geary ' s Division of greatly superior numbers who attempted to surprise him , will rank among the most distinguished feats of arms of this war . " The midnight Battle of Wauhatchie was followed in the next month by the brilliant victory at Lookout Mountain , where the 2nd Division fought its famous " battle above the clouds " . Geary was assisted in this engagement by Walter C . Whitaker ' s Brigade of the IV Corps . One of Whitaker ' s regiments , the 8th Kentucky , was the first to plant its flag on the summit of the mountain . Redesignation . In April 1864 , the designation of the corps was changed to that of the XX Corps . Generals Williams and Geary still retained command of their divisions and the men still wore their XII Corps badge . This badge ( a five - pointed star , or pentagram ) was adopted by the reorganized corps . The new organization was formed by the consolidation of the XI and XII corps , to which was added some minor commands . This action of the War Department was based on the small sizes of the two corps : the XI had been extremely damaged at Gettysburg and the XII had always been the smallest in the army . Nonetheless , the soldiers of the XII Corps were very upset at the loss of their original corps identity . Upon the discontinuance of the XII Corps , General Slocum was assigned to the command of the District of Vicksburg , but resumed the corps command — of the XX Corps — during the Atlanta Campaign , General Hooker having been relieved . Slocum afterwards commanded the Army of Georgia while on the March to the Sea and in the Carolinas Campaign . Command history . * Corps assigned to the Army of the Cumberland ; other entries assigned to the Army of the Potomac After April 18 , 1864 , the divisions of the XII Corps became part of the XX Corps .