The army 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"[1] 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On May 21, Cocke was replaced in command in northeastern Virginia by Milledge L. Bonham, who was in turn superseded by P. G. T. Beauregard ten days later.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 brigades (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.

[2] 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.