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I Corps (First Corps) was the designation of three different corps-sized units in the Union Army during the American Civil War. The units served in the following armies:

• Army of the Ohio/Army of the Cumberland, Alexander M. McCook, September 29, 1862 – November 5, 1862
• Army of the Mississippi, George W. Morgan, January 4, 1863 – January 12, 1863
• 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.

Union Army 1st Division Badge, I Corps

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. It was held in defense of Washington while the rest of the Army of the Potomac advanced to the Peninsula Campaign.

It 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 in the Battle of Antietam, under Major General Joseph Hooker. There, the division of Pennsylvania Reserves, under Brigadier General George G. Meade, took heavy casualties through its hard fighting, and was withdrawn to replenish.

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. He superbly led the corps through this battle, then through the Battle of Chancellorsville, where the I Corps was left in reserve.

Cap badge of the I Corps

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. 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.

* As III Corps, Army of Virginia

• Eicher, John H., & Eicher, David J.: Civil War High Commands, Stanford University Press, 2001, ISBN 0-8047-3641-3
• Fox, William F.: Regimental Losses in the American Civil War, Albany Publishing, 1889 (online text of I Corps section)