I Corps (Union Army)

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
* As III Corps, Army of Virginia