Army of the Potomac The Army of the Potomac was the major Union Army in the Eastern Theater of the American Civil War . History . 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 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 . Famous units . Because of its proximity to the large cities of the North , such as Washington , D . C . , Philadelphia , and New York City , the Army of the Potomac received more contemporary media coverage than the other Union field armies . Such coverage produced fame for a number of this army ' s units . Individual brigades , such as the Irish Brigade , the Philadelphia Brigade , the First New Jersey Brigade , the Vermont Brigade , and the Iron Brigade , all became well known to the general public , both during the Civil War and afterward . Corps . 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 .