The Army of the Potomac was created in 1861, but was then 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, Pennsylvania, and the Shenandoah. The men under Banks's command became an infantry division in the Army of the Potomac. 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. On the contrary, 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 -- Our Outlying Picket in the Woods, 1862 The Army of the Potomac underwent many structural changes during its existence. The army was divided by Ambrose 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. (Joseph 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 Ulysses S. 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. After the end of the war, it was disbanded on June 28, 1865, shortly following its participation in the Grand Review of the Armies. 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.
Beginning on March 13, 1862, President Abraham Lincoln established corps as the major subordinate units of the Army of the Potomac. (Up until this time, McClellan resisted the formation of corps, which had been prominent features of Napoleon's army, preferring to see how his division commanders fared in combat on the Peninsula before elevating them to higher command. Lincoln selected the corps commanders based on their seniority, without McClellan's approval.) The original corps were I Corps (McDowell), II Corps (Sumner), III Corps (Heintzelman), and IV Corps (Keyes). During the Peninsula Campaign, McClellan created two more, commanded by men he considered more loyal to him: V Corps (Porter) and VI Corps (Franklin). For the remainder of the war, corps were added and subtracted from the army. IV Corps headquarters and one division remained on the Virginia Peninsula. Those added to the Army of the Potomac were IX Corps, XI Corps (Sigel's I Corps in the former Army of Virginia), XII Corps (Banks's II Corps from the Army of Virginia), added in 1862; and the Cavalry Corps, created in 1863. Eight of these corps (seven infantry, one cavalry) served in the army during 1863, but due to attrition and transfers, the army was reorganized in March 1864 with only four corps: II, V, VI, and Cavalry. The IX Corps returned to the army in 1864, after being assigned to the West in 1863 and then serving alongside the Army of the Potomac in early 1864. Two divisions of the Cavalry Corps were transferred to the Shenandoah Valley, and the 2nd division alone remained under Meade's command. Later that division too was assigned to the cavalry command of MG Philip Sheridan for the closing campaigns of the war.