On July 16, therefore, McDowell began his advance, having with him about 35,000 men, and by the 19th he was at the stream of Bull Run, behind which the Confederates lay. He planned his battle skillfully, and began his attack on the morning of the 21st. On the other hand, Beauregard was at the double disadvantage of misapprehending his opponent’s purpose, and of failing to get his orders conveyed to his lieutenants until the fight was far advanced. The result was, that at the beginning of the afternoon the Federals had almost won a victory which they fully deserved. That they did not finally secure it was due to the inefficiency of General Patterson. This general had crossed the Potomac a few days before and had been instructed to watch Johnston, who had drawn back near Winchester, and either to prevent him from moving his force from the Shenandoah Valley to Manassas, or, failing this, to keep close to him and unite with McDowell. But Patterson neither detained nor followed his opponent. On July 18 Beauregard telegraphed to Johnston: “If you wish to help me, now is the time.” If Patterson wished to help McDowell, then, also, was the time. The Southern general seized his opportunity, and the Northern general let his opportunity go. Johnston, uninterrupted and unfollowed by Patterson, brought his troops in from Manassas Junction upon the right wing of the Federals at the very moment and crisis when the battle was actually in the process of going in their favor. Directly all was changed. Older troops would not have stood, and these untried ones were defeated as soon as they were attacked. Speedily retreat became rout, and rout became panic. At a great speed the frightened soldiers, resolved into a mere disorganized mob of individuals, made their way back to the camps on the Potomac; many thought Washington safer, and some did not stop short of their distant Northern homes.
The Southerners, who had been on the point of running away when the Northerners anticipated them in so doing, now triumphed immoderately, and uttered boastings magniloquent enough for Homeric heroes. Yet they were, as General Johnston said, “almost as much disorganized by victory as were the Federals by defeat.” Many of them also hastened to their homes, spreading everywhere the cheering tidings that the war was over and the South had won.