Bull Run campaign

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.

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 outnumbered.

During this time, Patterson was having difficulty receiving supplies from Pennsylvania; also his regiments of three–month volunteers were nearing the end of their enlistments and were refusing to stay any longer.

Due to Pressure from U.S. President Abraham Lincoln and the Union press, McDowell started his campaign against Manassas Junction on July 17.

[9] Following the Confederate abandonment of Harpers Ferry on June 15, Johnston sent Colonel 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.

Outnumbered and outflanked, Jackson fell back slowly, giving time for his supply train to escape before retreating to rejoin Johnston's main army.

A Confederate attack on the Union left flank in the late afternoon forced McDowell to retreat back to the defenses of Washington, D.C.[13] Davis arrived on the Manassas battlefield soon after the battle ended.