The Army of the Potomac was created in 1861, but was only the size of a corps (relative to the size of Union armies later in the war). Its nucleus was called the Army of Northeastern Virginia, under Brig. Gen. Irvin McDowell, and it was the army that fought (and lost) the war's first major battle, the First Battle of Bull Run. The arrival in Washington, D.C., of Maj. Gen. George B. McClellan dramatically changed the makeup of that army. McClellan's original assignment was to command the Division of the Potomac, which included the Department of Northeast Virginia under McDowell and the Department of Washington under Brig. Gen. Joseph K. Mansfield. On July 26, 1861, the Department of the Shenandoah, commanded by Maj. Gen. Nathaniel P. Banks, was merged with McClellan's departments and on that day, McClellan formed the Army of the Potomac, which was composed of all military forces in the former Departments of Northeastern Virginia, Washington, Baltimore, and the Shenandoah. The men under Banks's command became an infantry division in the Army of the Potomac.[1] The army started with four corps, but these were divided during the Peninsula Campaign to produce two more. After the Second Battle of Bull Run, the Army of the Potomac absorbed the units that had served under Maj. Gen. John Pope.

It is a popular, but mistaken, belief that John Pope commanded the Army of the Potomac in the summer of 1862 after McClellan's unsuccessful Peninsula Campaign. However, Pope's army consisted of different units and was named the Army of Virginia. During the time that the Army of Virginia existed, the Army of the Potomac was headquartered on the Virginia Peninsula, and then outside Washington, D.C., with McClellan still in command, although three corps of the Army of the Potomac were sent to northern Virginia and were under Pope's operational control during the Northern Virginia Campaign.

The Army of the Potomac underwent many structural changes during its existence. The army was divided by Burnside into three Grand Divisions of two corps each with a Reserve composed of two more. Hooker abolished the Grand Divisions. Thereafter the individual corps, seven of which remained in Virginia, reported directly to army headquarters. (Hooker also created a Cavalry Corps by combining units that previously had served as smaller formations.) In late 1863, two corps were sent West, and—in 1864—the remaining five corps were recombined into three. Burnside's IX Corps, which accompanied the army at the start of Grant's Overland Campaign, was added later. For more detail, see the section Corps below.

The Army of the Potomac fought in most of the Eastern Theater campaigns, primarily in (Eastern) Virginia, Maryland, and Pennsylvania, and was disbanded on June 28, 1865, after the end of the war.

The Army of the Potomac was also the name given to General P.G.T. Beauregard's Confederate army during the early stages of the war (namely, First Bull Run; thus, the losing Union Army ended up adopting the name of the winning Confederate army). However, the name was eventually changed to the Army of Northern Virginia, which became famous under General Robert E. Lee.

George Brinton McClellan (December 3, 1826 – October 29, 1885) was a major general during the American Civil War. He organized the famous Army of the Potomac and served briefly (November 1861 to March 1862) as the general-in-chief of the Union Army. Early in the war, McClellan played an important role in raising a well-trained and organized army for the Union. However, although McClellan was meticulous in his planning and preparations, these attributes may have hampered his ability to challenge aggressive opponents in a fast-moving battlefield environment. He chronically overestimated the strength of enemy units and was reluctant to apply principles of mass, frequently leaving large portions of his army unengaged at decisive points.

McClellan's Peninsula Campaign in 1862 ended in failure, with retreats from attacks by General Johnston, but the emergence of General Robert E. Lee changed the character of the campaign and turned it into a humiliating Union defeat.

The Northern Virginia Campaign, also known as the Second Bull Run Campaign or Second Manassas Campaign, was a series of battles fought in Virginia during August and September 1862 in the Eastern Theater of the American Civil War. Confederate General Robert E. Lee followed up his successes of the Seven Days Battles in the Peninsula Campaign by moving north toward Washington, D.C., and defeating Maj. Gen. John Pope and his Army of Virginia.

Concerned that Pope's army would combine forces with Maj. Gen. George B. McClellan's Army of the Potomac and overwhelm him, Lee sent Maj. Gen. Thomas J. "Stonewall" Jackson north to intercept Pope's advance toward Gordonsville. The two forces initially clashed at Cedar Mountain on August 9, a Confederate victory. Lee determined that McClellan's army on the Virginia Peninsula was no longer a threat to Richmond and sent most of the rest of his army, Maj. Gen. James Longstreet's command, following Jackson. Jackson conducted a wide-ranging maneuver around Pope's right flank, seizing the large supply depot in Pope's rear, at Manassas Junction, placing his force between Pope and Washington, D.C. Moving to a very defensible position near the battleground of the 1861 First Battle of Bull Run, Jackson successfully repulsed Union assaults on August 29 as Lee and Longstreet's command arrived on the battlefield. On August 30, Pope attacked again, but was surprised to be caught between attacks by Longstreet and Jackson, and was forced to withdraw with heavy losses. The campaign concluded with another flanking maneuver by Jackson, which Pope defeated at the Battle of Chantilly on September

The Battle of Antietam (also known as the Battle of Sharpsburg, particularly in the South), fought on September 17, 1862, near Sharpsburg, Maryland, and Antietam Creek, as part of the Maryland Campaign, was the first major battle in the American Civil War to take place on Northern soil. It was the bloodiest single-day battle in American history, with almost 23,000 casualties.[1]

After pursuing Confederate General Robert E. Lee into Maryland, Union Army Maj. Gen. George B. McClellan launched attacks against Lee's army, in defensive positions behind Antietam Creek. At dawn on September 17, Maj. Gen. Joseph Hooker's corps mounted a powerful assault on Lee's left flank. Attacks and counterattacks swept across Miller's cornfield and fighting swirled around the Dunker Church. Union assaults against the Sunken Road eventually pierced the Confederate center, but the Federal advantage was not followed up. In the afternoon, Maj. Gen. Ambrose Burnside's corps entered the action, capturing a stone bridge over Antietam Creek and advancing against the Confederate right. At a crucial moment, Maj. Gen. A.P. Hill's division arrived from Harpers Ferry and launched a surprise counterattack, driving back Burnside and ending the battle. Although outnumbered two-to-one, Lee committed his entire force, while McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill. During the night, both armies consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan throughout September 18, while removing his wounded south of the river.[2]

Despite having superiority of numbers, McClellan's attacks failed to achieve concentration of mass, allowing Lee to counter by shifting forces along interior lines to meet each challenge. Despite ample reserve forces that could have been deployed to exploit localized successes, McClellan failed to destroy Lee's army. Nevertheless, Lee's invasion of Maryland was ended, and he was able to withdraw his army back to Virginia without interference from the cautious McClellan. Although the battle was tactically inconclusive, it had unique significance as enough of a victory to give President Abraham Lincoln the confidence to announce his Emancipation