Union Brig. Gen. Irvin McDowell and his 35,000-man army left Washington, D.C., on 16 July 1861 and moved toward Manassas Junction, a strategically important railroad intersection defended by Confederate Brig. Gen. P.G.T. Beauregard. The Union vanguard reached Centreville on 18 July, advanced southward to Blackburn's Ford on Bull Run, and retreated after a sharp skirmish. McDowell, headquartered at Centreville, revised his battle plan and on 21 July moved northwest to attack the Confederate left flank. After a day of intense fighting, Confederate reinforcements arrived on the field and repulsed the Federals, who, along with civilian observers, fled back through Centreville toward Washington.
