I Corps (ACW)

I Corps
I Corps badge (circle)
Active	1862–1864
Country	United States of America
Branch	Army
Type	Army Corps
Size	Corps
Engagements	American Civil War
Commanders
Notable commanders	Irvin McDowell Joseph Hooker John F. Reynolds † John Newton
Insignia
1st Division
2nd Division
3rd Division

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 describe the First Veteran Corps from 1864 to 1866.

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.