In 1861, when the opposing Armies of the Potomac came into being (in the first year, both the North and South called their armies by the same name), the armies were a loose organizations of divisions. When it became clear that this would not be a short war, the size of the armies swelled to such an extent that a better organization was needed to relieve the army commander of detailed matters on a grand tactical level.

In March 1862, George B. McClellan reorganized the Union armies. Divisions were organized into Corps, with three divisions per corps. Each Corps would be commanded by a major general and designated by a roman numeral. Armies would comprise various numbers of corps. Thus reorganized, the Army of the Potomac would contain five corps, while a year later it would have seven. This organization continued as the basic Union command structure until the end of the war. In all, the Union would create 14 armies and 43 corps during the war.

Until 6 November 1862, the renamed Army of Northern Virginia would not officially have corps since Confederate law did not recognize units larger than a division. Robert E. Lee, however, had gone to the corps system after the Second Battle of Manassas, basically using the existing wings of Longstreet and Jackson as the First and Second corps.

Confederate corps were known by the name of the corps commander. Each would comprise four or five divisions. In 1863, following Jackson's death at Chancellorsville, Lee created a third corps to increase the flexibility lost with Jackson. During the Gettysburg campaign, Longstreet, Ewell, and A. P. Hill would lead the corps.

During the course of the war, different corps would distinguish themselves in various ways.

Union

For the Union, the following corps stand out:

I Corps

In March 1862, the divisions led by William Franklin, George McCall, and Rufus King were organized into the I Corps. From 4 April to 12 September 1862, it became part of McDowell's Department of the Rappahannock. Then, with the 1862 Confederate invasion of Maryland, the reorganized I Corps rejoined the Army of the Potomac. Under the leadership of Joseph Hooker, it took part at South Mountain and saw heavy action at Antietam where it overran Jackson in the Cornfield, only to have its advance halted when John Hood counterattacked.

Hooker would be wounded during the battle. Meade's Pennsylvania Reserves especially distinguished themselves during the Antietam campaign. I Corps did not take center stage again until Gettysburg. There, first under Reynolds and then under Doubleday when Reynolds died, the Corps played an important role during the first day of the battle. Its stands on McPherson's Ridge and Seminary Ridge gave Meade time to secure his defensive line. Led by Newton for the remaining two days of Gettysburg, it took part in repulsing Pickett's Charge. But Gettysburg decimated I Corps. It took part in the battle at Mine's Run, but was no longer a corps. On 23 March 1864, it was officially absorbed into V Corps. It had proved to be one of the best corps in the Union army, with its extremely high casualty rate attesting to its tenacity and fighting spirit.

II Corps

Edwin Sumner was given command of Il Corps when it was created in March 1862. During the Peninsular campaign, II Corps distinguished itself at the Battle of the Seven Pines. The Corps suffered heavy losses when it was committed piecemeal by Sumner during the Antietam battle. Under much criticism, Sumner resigned his command after the battle. Darius Couch became the commander and led it through Fredericksburg, where it assaulted Marye's Heights and was repeatedly beaten back.

During the Chancellorsville Campaign, II Corps fought well, repulsing Confederate attacks until being withdrawn on 3 May 1863. Couch, disgusted with Hooker, asked for and received a transfer. Winfield Scott Hancock took command and led the Corps during its greatest battle, Gettysburg. It was there that the Corps thwarted Lee's attempt to turn the left flank on the second day and withstood Pickett's Charge on the third day.

Confederates

Of the Confederate corps, the first two corps were the most memorable:

Longstreet's Corps

Longstreet's Corps officially became a corps on 6 November 1862, although it had fought together throughout the Antietam campaign. It fought in every major campaign in the east, as well as at Chickamauga. It contained some of the most famous troops of the war - Hood's Texas Brigade, 1st Virginia Regiment, Kershaw's South Carolinians, and the Washington Artillery of New Orleans. It was Pickett's division that made the famous charge at Gettysburg. The Corps would continue to fight well throughout the war, holding the Confederate left at the siege of Petersburg and fighting in the retreat that ended at Appomattox Court House.

Second Corps

Originally, this was Jackson's corps, but after his death, it was commanded by Ewell, Early, and finally by John Gordon. The Corps was renowned for its aggressiveness and its flexibility of maneuver under Jackson. During the Gettysburg campaign, especially the first day of the battle, Second Corps fought well. However, at Kelly's Ford and at Rappahannock Bridge, the Corps tasted defeat for the first time.

In 1864, Second Corps attacked Grant in the Wilderness, only to be nearly destroyed when it was overrun in the Muleshoe at Spotsylvania. Most of the Stonewall Division was captured during this battle. The Corps' last moments of glory came under the leadership of Jubal Early during the summer of 1864, when they swept north from Lynchburg to within sight of Washington. Later, in the Shenandoah Valley, it would be defeated by Sheridan at Winchester, Fisher's Hill, and Cedar Creek. By the end of the war, this proud corps had been ground down to less than 6,000 men.