Confederate Army of the Potomac The Confederate Army of the Potomac , whose name was short - lived , was the command under Brig . Gen . P . G . T . Beauregard in the early days of the American Civil War . Its only major combat action was the First Battle of Bull Run . Afterwards , the Army of the Shenandoah was merged into the Army of the Potomac with Gen . Joseph E . Johnston , the commander of the Shenandoah , taking command . The Army of the Potomac was renamed the Army of Northern Virginia on March 14 , 1862 , with Beauregard ' s original army eventually becoming the First Corps , Army of Northern Virginia . History . The was formed from Confederate units defending northeastern Virginia , which arrived over the course of April to July 1861 . Philip St . George Cocke was appointed to command the area of Virginia along " the line of the Potomac " and to muster the local militia companies into Confederate service . Regiments from Alabama , Georgia , Louisiana , Mississippi , and South Carolina also arrived in Virginia and were assigned to Cocke ' s command . On May 21 , Cocke was replaced in command in northeastern Virginia by Milledge L . Bonham , who was in turn superseded by P . G . T . Beauregard ten days later . Beauregard divided his army into six brigade s ( two of which were commanded by Cocke and Bonham ) and concentrated them along the south bank of Bull Run , intending to defend the rail center of Manassas Junction . He would be reinforced by additional regiment s over the next few weeks , forming a seventh brigade , and received the brigade of Theophilus Holmes as well . Over the course of his first weeks in command , Beauregard sent to Confederate president Jefferson Davis various plans for an offensive against Union forces in northern Virginia , which usually involved coordination with Joseph E . Johnston ' s Army of the Shenandoah . Both Davis and his military advisor , Robert E . Lee , rejected these plans as being impractical . On July 18 , as forces from the Union Department of Northeastern Virginia commanded by Irvin McDowell advanced to within a few miles of Beauregard ' s positions , the Confederate War Department ordered Johnston to transfer his army to reinforce Beauregard ; his army arrived by rail over the next few days . Johnston was the senior officer present and had overall command of the combined Confederate armies . Beauregard had drawn up plans for an attack on the Union left wing across Bull Run and convinced Johnston to approve the plan . Johnston deferred the issuing of orders to Beauregard since he was more familiar with the terrain . During the First Battle of Bull Run , fought on July 21 , the opening stages of the fighting on Matthews Hill was done by Colonel Nathan Evans ' brigade from the Army of the Potomac , reinforced by two brigades from Johnston ' s army . As the fighting shifted to Henry House Hill , Cocke ' s and Jubal Early ' s brigades along with units from Bonham ' s brigade from Beauregard ' s army , along with the rest of Johnston ' s army , were shifted to the Confederate left wing . An attack by Early ' s brigade on the Union right flank at about 4 p . m . helped drive the Union army from the field . Beauregard ' s brigades lost fewer men than Johnston ' s brigades , but some commands still suffered a casualty rate as high as twenty percent . Most of Beauregard ' s brigades remained on the Confederate right , since Beauregard still hoped to launch an attack on the Union left wing . However , because of contradictory and confusing orders , some of which apparently were never delivered to the intended recipients , the attack was never launched . The brigades on this part of the field never fought at all or participated in only minor skirmishing . David R . Jones and James Longstreet crossed Bull Run and attempted to organize an attack but following a brief skirmish Jones ' brigade was thrown back due to heavy artillery fire . Both brigades subsequently withdrew back to Bull Run . The two Confederate armies were consolidated into a single army following the battle , retaining the name " Army of the Potomac " and with Johnston in command ; Beauregard was initially posted as its second - in - command but he was soon transferred to the Western Theater . The army was spread through northern Virginia to observe the Union Army of the Potomac in Washington , fighting several small skirmishes including the Battle of Ball ' s Bluff . In the spring of 1862 , Johnston ' s army was transferred to the Richmond area , where the Army of the Peninsula and the Confederate garrison of Norfolk , Virginia , were incorporated into the army . At this time in the Peninsula Campaign , the army was officially renamed the Army of Northern Virginia , although Johnston continued to use the name Army of the Potomac until he was wounded .