Explanation: In the North, people clamored for an immediate invasion to crush the rebellious South. Northern politicians insisted the Confederate Congress must not be allowed to convene in their new capitol, Richmond, VA. on July 20th. President Lincoln bowed to public pressure, and ordered a premature advance.
While proffessional officers urged patience, President Lincoln, bowing to public pressure, ordered a premature advance on Virginia. He assigned the task to Brig, Gen. Irvin McDowell, commander of the principal Union army encircling Washington, D.C. While McDowell advanced across northern Virginia, Maj. Gen. Robert Patterson, with 18,000 men, was directed to prevent Confederates in the Shenandoah Valley from reinforcing the army facing McDowell.
On July 16, McDowell's 35,000-man army marched out of Washington. Two days of confusion, straggling, and a snail-like pace followed before the Federals entered Centreville. Behind this muddy, sluggish stream lay Gen. P.G.T. Beauregard's Army of the Potomac, 22,000 Confederates, protecting the strategically vital railroad intersection of Manassas Junction.
When McDowell arrived at Centreville, he ordered a reconnaissance in force that same day. This probe by Brig. Gen. Daniel Tyler's division resulted in a brief, sharp engagement with 2 Confederate brigades at Blackburn's Ford. McDowell spent 2 more crucial days reconnoitering, reissuing ammunition and rations lost or consumed by his ill-disciplined troops, and preparing a battle plan.
These delays also permitted the Confederates to combine their forces. In the Shenandoah Valley opposing Patterson was Brig. Gen. Joseph E. Johnston and 12,000 Confederates. When Confederate authorities learned through spies of McDowell's advance, they had ordered Johnston east to reinforce Beauregard. Screened by a cavalry brigade, Johnston abandoned his lines early on the 18th and marched to Piedmont, where his troops loaded the cars of the Manassas Gap Railroad- the first time in military history that a railroad was used to achieve strategic mobility. A befuddled Patterson did not learn of Johnston's departure until the 20th, and by then all but one of the brigades had joined Beauregard.
The long-awaited showdown finally occured Sunday, July 21. In a battle fought basically between 2 armed mobs of green soldiers, the Confederates, on the defensive, had the best of it, and the day ended at the climax, urging a pursuit to Washington, but the victorious Confederates were as disorganized as the fleeing Union army.
The campaign had a profound impact on the country. Southerners became overconfident and tended to relax in an atmosphere of overconfidence. The Northerners became grimly determined and was spurred to greater effort because of the defeat at Bull Run. The battle, which brought carriages filled with festive spectators from Washington, confirmed realists' insistence that this war would be neither romantic nor brief. Nevertheless, both sides spent the remainder of 1861 in earnestly preparing for a hard war. Six days after the defeat, Lincoln replaced McDowell with Maj. Gen. George B. McClellan, a popular career soldier he believed could lead the Union to victories on the battlefield.
The number of casualties is difficult to determine, but a fair estimate puts the Union loss at 500 killed, 1,000 wounded, and 1,200 missing and the Confederate loss at 400 killed, 1,600 wounded, and 13 missing. The wide variety of uniforms worn by participants in the battle had caused much confusion, which led subsequently to the adoption of a gray uniform for Confederate troops and blue for the Federals.