I Corps ( Union Army ) < br > I Corps ( First Corps ) was the designation of three different corps - sized units in the Union Army during the American Civil War . Separate formation called the I Corps served in the Army of the Ohio / Army of the Cumberland under Alexander M . McCook from September 29 , 1862 to November 5 , 1862 , in the Army of the Mississippi under George W . Morgan from January 4 , 1863 to January 12 , 1863 , and in the Army of the Potomac and Army of Virginia ( see below ) . The first two were units of very limited life ; the third was one of the most distinguished and veteran corps in the entire Union Army , commanded by very distinguished officers . The term " First Corps " is also used to described the First Veteran Corps from 1864 to 1866 . History . The I Corps was activated March 13 , 1862 , when President Abraham Lincoln ordered the creation of a four - corps army , then under the command of Major General George B . McClellan . The first commander of this corps was Major General Irvin McDowell and it contained three divisions . Originally intended to go to the Peninsula Campaign with the rest of the army , it was instead detached and left in the Fredericksburg area after Stonewall Jackson ' s actions in the Shenandoah Valley caused the Lincoln Administration to fear for Washington ' s safety . One of its divisions , the Pennsylvania Reserves , was eventually sent to join the main army in June . Temporarily attached to the V Corps , it saw heavy action at Gaines ' Mill and Glendale . Division commander Brig . Gen . George McCall and future I Corps commander Brig . Gen . John Reynolds were both captured and freed in a prisoner exchange that August . The Pennsylvania Reserves rejoined the I Corps after the Seven Days Battles , and the outfit was then consolidated into the Army of Virginia under Major General John Pope , and fought in the Second Battle of Bull Run , as the Third Corps , Army of Virginia . Afterwards , its name was restored . It rejoined the Army of the Potomac and crossed the Potomac River into Maryland to fight at South Mountain and the Battle of Antietam , under Major General Joseph Hooker . John Reynolds ( who had been elevated to division command of the Reserves ) was temporarily detached to train militia troops in his home state of Pennsylvania and did not participate in the Maryland Campaign . At Antietam , the I Corps was among the first troops to fight and suffered enormous losses in the battles around the cornfield and Dunker Church . Hooker was wounded in the foot during the battle and command of the I Corps devolved on Meade ( the ranking division commander ) . In October , Reynolds returned and was made commander of the corps . Having fought three battles in six weeks , the I Corps was severely depleted . An influx of new volunteer regiments ( both three year and nine month ) arrived to replenish its ranks , and by November it was back up to full strength . The corps moved southward to fight General Robert E . Lee ' s army at the Battle of Fredericksburg , commanded by Major General John F . Reynolds , arguably the best Union corps commander in the Eastern Theater . At Fredericksburg , Meade and John Gibbon ' s divisions fought Stonewall Jackson ' s corps south of the town while Doubleday ' s division was held in reserve . The I Corps did not see any significant action in the Chancellorsville Campaign . In its last major battle , the Battle of Gettysburg , General Reynolds was killed just as the first troops arrived on the field , and command was inherited by Major General Abner Doubleday . Although putting up a ferocious fight , the I Corps was overwhelmed by the Confederate Third Corps ( A . P . Hill ) and Robert E . Rodes ' s division of Richard S . Ewell ' s Second Corps . It was forced to retreat through the town of Gettysburg , taking up defensive positions on Cemetery Hill after the 16th Maine ' s brave stand of which only 39 soldiers returned . The next day ( July 2 , 1863 ) , the command was given to Major General John Newton , a division commander from the VI Corps . This was a controversial move that deeply offended the more senior Doubleday . Newton led it through the remainder of the battle , including the defense against Pickett ' s Charge , and through the Mine Run Campaign that fall . On March 24 , 1864 , the Civil War career of the I Corps came to an end as it was disbanded and its depleted units were reorganized into two divisions , which were transferred into the V Corps of the Army of the Potomac . Command history . < nowiki > * < / nowiki > As III Corps , Army of Virginia