Manassas Campaign The Manassas Campaign was a series of military engagements in the Eastern Theater of the American Civil War . Background . The Confederate forces in northern Virginia were orgainized into two field armies . Brigadier General P . G . T . Beauregard was appointed commander of the Confederate Army of the Potomac in northeastern Virginia to defend the rail center of Manassas Junction ; while General Joseph E . Johnston commanded the Army of the Shenandoah near Harpers Ferry in the Shenandoah Valley . The Manassas Gap Railroad connected the two forces and allowed for the quick transfer of reinforcements between the two armeis . During the months of June and July , Beauregard sent Confederate President Jefferson Davis several proposals for offensive operations into Maryland , involving the various Confederate armies in Virginia , but Davis rejected them for being impractical , saying the Confederates lacked the proper resources to support any of his offensive plans . Following the Union occupation of Alexandria , Virginia , the Union forces in Virginia were orgainized into the Department of Northeastern Virginia , commanded by Brigadier General Irvin McDowell , who was ordered to advance on the Confederate national capital of Richmond , Virginia . Meanwhile , Robert Patterson was assigned to command the Department of Pennsylvania and was ordered to tie down Johnston ' s forces in the northern Shenandoah Valley , preventing him from reinforcing Beauregard . Patterson advanced into the Valley in early June , forcing the evacuation of Harpers Ferry on June 17 . Johnston retreated back to the town of Winchester , where he was reinforced by home guard units and local militia , which caused Patterson to think he was outhumbered . During this time , Patterson was having difficulty receiving supplies from Pennsylvania ; also his regiments of three – month volunteers wre nearing the end of their enlistments and were refusing to stay any longer . The Union general – in – chief , Brevet Lieutenant General Winfield Scott , was pressing him to send his Regular Army units to McDowell ' s army . During the months of June and early July , the armies of McDowell and Beauregard engaged in several skirmishes in northeastern Virginia while the Union government and military leadership debated the proper course of action for McDowell to take . Scott favored concentrating a Union army to capture the Mississippi River valley , while McDowell viewed his army as too inexperienced to attack Beauregard yet . Due to Pressure from U . S . President Abraham Lincoln and the Union press , McDowell started his campaign against Manassas Junction on July 17 . Johnston received orders the next day to start transferring his army to Manassas Junction to reinforce Beauregard ; using the Manassas Gap Railroad , his army arrived on July 20 and 21 . Battles . Hoke ' s Run . Following the Confederate abandonment of Harpers Ferry on June 15 , Johnston sent Brigadier General Thomas J . Jackson ' s brigade to establish a camp near the town of Martinsburg , so to both serve as a warning of a Union advance and to delay Patterson ' s force . On July 2 , Patterson crossed the Potomac and advanced on Jackson ' s position . Outnumbereed and outflanked , Jackson fell back slowly , giving time for his supply train to escape before retreating to rejoin Johnston ' s main army . Blackburn ' s Ford . McDowell ' s army arrived at Centerville during the morning of July 18 , lead by Brigadier General David Tyler ' s division . Under orders to reconnoiter the Confederate defenses near Blackburn ' s Ford but not to bring on an engagement , Tyler launched an attack with Colonel Israel B . Richardson ' s brigade but was repulsed by the brigades of James Lonstreet and Jubal Early . The battle ended with an artillery duel which lasted until dark , at which time Tyler withdrew back to Centerville . First Bull Run ( Manassas ) . Following Tyler ' s defeat , McDowell decided to attempt an attack on the Confederate left flank . Starting about dawn on the 21 , one Union division started a diversionary attack on the Stone Bridge on the Confederate left and two other divisions demonstrated against the Confederate center and right , while two other divisions marched around the Confederate left flank to launch an attack on the Confederate rear . The Union flanking column was spotted and Confederate reinforcements were rushed to Matthew ' s Hill but were driven back to Henry House Hill . A new Confederate defensive line , using brigades from Johnston ' s army , was formed along the hill , where they held against repeated Union attacks during the afternoon . A Confederate attack on the Union left flank in the late afternoon forced McDowell to retreat back to the defenses of Washington , D . C . Aftermath . Davis arrived on the Manassas battlefield soon after the battle ended . Although he attempted to orgainize a pursuite of the Union army , he was convinced that the Confederate armies were too disorgainized to mount an effective pursuit . The dispute between Beauregard and Davis over who was responsible for the failed pursuit resulted in Beauregard ' s transfer to the Western Theater . In October , Johnston ' s and Beauregard ' s commands were merged into the Department of Northern Virginia , with the combined army retaining the name " Army of the Potomac " . Following First Bull Run , McDowell retreated to Centerville and set up a rearguard . At a council of war held after sunset , a majority of officers urged a retreat , which started that night . In August , the Department of Northeastern Virginia was merged with other departments in Maryland to form the Department of the Potomac , commanded by George C . McClellan . McDowell was demoted to commanding a division . Patterson was blamed for allowing Johnston to reinforce Beauregard and was also relieved of command .