First Battle of Bull Run - Background

Background

Further information: Confederate order of battle, Union order of battle

Brig. Gen. Irvin McDowell was appointed by President Abraham Lincoln to command the Army of Northeastern Virginia. Once in this capacity, McDowell was harassed by impatient politicians and citizens in Washington, who wished to see a quick battlefield victory over the Confederate Army in northern Virginia. McDowell, however, was concerned about the untried nature of his army. He was reassured by President Lincoln, "You are green, it is true, but they are green also; you are all green alike." Against his better judgment, McDowell commenced campaigning. On July 16, 1861, the general departed Washington with the largest field army yet gathered on the North American continent, about 35,000 men (28,452 effectives). McDowell's plan was to move westward in three columns, make a diversionary attack on the Confederate line at Bull Run with two columns, while the third column moved around the Confederates' right flank to the south, cutting the railroad to Richmond and threatening the rear of the rebel army. He assumed that the Confederates would be forced to abandon Manassas Junction and fall back to the Rappahannock River, the next defensible line in Virginia, which would relieve some of the pressure on the U.S. capital.

The Confederate Army of the Potomac (21,883 effectives) under Beauregard was encamped near Manassas Junction, approximately 25 miles (40 km) from the United States capital. McDowell planned to attack this numerically inferior enemy army. Union Maj. Gen. Robert Patterson's 18,000 men engaged Johnston's force (the Army of the Shenandoah at 8,884 effectives, augmented by Maj. Gen. Theophilus H. Holmes's brigade of 1,465) in the Shenandoah Valley, preventing them from reinforcing Beauregard.

After two days of marching slowly in the sweltering heat, the Union army was allowed to rest in Centreville. McDowell reduced the size of his army to approximately 30,000 by dispatching Brig. Gen. Theodore Runyon with 5,000 troops to protect the army's rear. In the meantime, McDowell searched for a way to outflank Beauregard, who had drawn up his lines along Bull Run. On July 18, the Union commander sent a division under Brig. Gen. Daniel Tyler to pass on the Confederate right (southeast) flank. Tyler was drawn into a skirmish at Blackburn's Ford over Bull Run and made no headway.

Becoming more frustrated, McDowell resolved to attack the Confederate left (northwest) flank instead. He planned to attack with Brig. Gen. Daniel Tyler's division at the Stone Bridge on the Warrenton Turnpike and send the divisions of Brig. Gens. David Hunter and Samuel P. Heintzelman over Sudley Springs Ford. From here, these divisions could march into the Confederate rear. The brigade of Col. Israel B. Richardson (Tyler's Division) would harass the enemy at Blackburn's Ford, preventing them from thwarting the main attack. Patterson would tie down Johnston in the Shenandoah Valley so that reinforcements could not reach the area. Although McDowell had arrived at a theoretically sound plan, it had a number of flaws: it was one that required synchronized execution of troop movements and attacks, skills that had not been developed in the nascent army; it relied on actions by Patterson that he had already failed to take; finally, McDowell had delayed long enough that Johnston's Valley force was able to board trains at Piedmont Station and rush to Manassas Junction to reinforce Beauregard's men.

On July 19–20, significant reinforcements bolstered the Confederate lines behind Bull Run. Johnston arrived with all of his army, except for the troops of Brig. Gen. Kirby Smith, who were still in transit. Most of the new arrivals were posted in the vicinity of Blackburn's Ford and Beauregard's plan was to attack from there to the north toward Centreville. Johnston, the senior officer, approved the plan. If both of the armies had been able to execute their plans simultaneously, it would have resulted in a mutual counterclockwise movement as they attacked each other's left flank.

McDowell was getting contradictory information from his intelligence agents, so he called for the balloon Enterprise, which was being demonstrated by Prof. Thaddeus S. C. Lowe in Washington, to perform aerial reconnaissance.