The Confederate army at Manassas, commanded by General Beauregard, was 21,000 strong. The Union army under General McDowell was 30,000 strong, with 10,000 more reserves available. Unfortunately, McDowell was the first of many Union commanders in Virginia who moved too slowly. By the time he reached the Confederate position, another 12,000 Confederates were arriving from the Shenandoah Valley. On 21 July the two armies clashed in the first major battle of the war, known as either Bull Run or Manassas. After attacking unsuccessfully for most of the day, the untrained Union army collapsed and fled after one of the newly arrived Confederate units launched a counter attack.
The Union army returned to Washington with much more speed than they had left. Once within the fortifications of the capitol, the army regrouped and prepared for any Confederate attack. None came, perhaps wisely given the strength of the fortifications and the damage done to the Confederate army. Bull Run/ Manassas ended the serious fighting in Virginia for the year. It had many long lasting effects. In the South, it generated a dangerous belief in the superiority of the Confederate soldier which was to lead later Generals into some very rash decisions. In the North it generated a determination not to be beaten that manifested itself in the ease with which Lincoln was able to pass laws that would create a million man army.
Defeat at Bull Run cost McDowell his command. In his place was put General George McClellan, already a victorious commander in West Virginia. Regardless of his later flaws, McClellan turned out to be a very talented creator of armies. As later fighting was to show, the Army of the Potomac that he created was able to hold its own against Lee’s Army of North Virginia. McClellan was also hugely popular at every level in the army, something that was to help preserve him in his post after his limits as a battlefield commander had become glaringly obvious.