The First Battle of Bull Run was the first major battle of the Civil War and gave the country a taste of what was to come in the way of casualties. Months after the capture of Fort Sumter, the Union leadership was pressured into a significant battle against the Confederacy.
They believed that the army would be able to march into Virginia and quickly take Richmond, but that would not be the case. The Union soldiers were not well-trained and were too green for battle, and the end result would be a Confederate victory at the expense of many casualties.
On July 19–20, significant reinforcements bolstered the Confederate lines behind Bull Run. Joseph E. Johnston arrived with all of his men except for the troops of Brig. Gen. Kirby Smith, who was 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.
McDowell's Army of Northeastern Virginia was organized into five infantry divisions of three to five brigades each. Each brigade contained three to five infantry regiments. An artillery battery was generally assigned to each brigade.
The total number of Union troops present at the Battle of First Bull Run was about 35,000, although only about 18,000 were actually engaged. The Union army was organized as follows:
- 1st Division of Brig. Gen. Daniel Tyler, the largest in the army, contained four brigades led by Brig. Gen. Robert C. Schenck, Col. Erasmus Keyes, Col. William T. Sherman, and Col. Israel B. Richardson;
- 2nd Division of Col. David Hunter of two brigades. These were led by Cols. Andrew Porter and Ambrose E. Burnside;
- 3rd Division of Col. Samuel P. Heintzelman included 3 brigades led by Cols. William B. Franklin, Orlando B. Willcox, and Oliver O. Howard;
- 4th Division of Brig. Gen. Theodore Runyon, without brigade organization and not engaged, contained seven regiments of New Jersey and one regiment of New York volunteer infantries;
- 5th Division of Col. Dixon S. Miles included 2 brigades commanded by Cols. Louis Blenker and Thomas A. Davies;
While McDowell organized the Army of Northeastern Virginia, a smaller Union command was organized and stationed northwest of Washington, near Harper's Ferry.
Commanded by Maj. Gen. Robert Patterson, 18,000 men of the Department of Pennsylvania protected against a Confederate incursion from the Shenandoah Valley
Joseph E. Johnston's forces were organized as follows:
- The Army of the Potomac (Brig. Gen. P. G. T. Beauregard) was organized into six infantry brigades, with each brigade containing three to six infantry regiments. Artillery batteries were assigned to various infantry brigades. The total number of troops in the Confederate Army of the Potomac was approximately 22,000. Beauregard's army also contained thirty-nine pieces of field artillery and a regiment of Virginia cavalry. The Army of the Potomac was organized into seven infantry brigades. These were:
- 1st Brigade, under Brig. Gen. Milledge Luke Bonham;
- 2nd Brigade, under Brig. Gen. Richard S. Ewell;
- 3rd Brigade, under Brig. Gen. David R. Jones;
- 4th Brigade, under Brig. Gen. James Longstreet; Young artillery lieutenant Edward Porter Alexander served in this brigade.
- 5th Brigade, under Col. Philip St. George Cocke;
- 6th Brigade, under Col. Jubal Early;
- 7th Brigade, under Col. Nathan G. Evans.
- Reserve Brigade, under Brig. Gen. Theophilus H. Holmes
- The Army of the Shenandoah (Brig. Gen. Joseph E. Johnston) was also organized into brigades. It consisted of four brigades of three to five infantry regiments each, which totaled approximately 12,000 men. Each brigade was assigned one artillery battery. In addition to the infantry, there were twenty pieces of artillery and about 300 Virginia cavalrymen under Col. J. E. B. Stuart. Although the combined strength of both Confederate armies was about 34,000, only about 18,000 were actually engaged at First Bull Run. The Army of the Shenandoah consisted of four infantry brigades:
- 1st Brigade, commanded by Brig. Gen. Thomas J. Jackson;
- 2nd Brigade, commanded by Col. Francis S. Bartow;
- 3rd Brigade, commanded by Brig. Gen. Barnard E. Bee;
- 4th Brigade, commanded by Brig. Gen. Edmund Kirby Smith.
The two opposing generals planned to outflank their opponent’s left but were slowed for different reasons:
- The Confederate troops suffered from communication issues and a lack of coordination between units.
- The Union troops suffered from a complicated plan that was too complex for the Green troops to execute.
Breaking the tie between the two was the excellent scouting on the Confederate side. The correct intelligence proved to be the advantage the Confederates needed to carry the morning.
Irvin McDowell's troops began shelling the Confederate across Bull Run while other Union troops initiated an attack on the Confederate left flank. Beauregard began to execute a similar strategy on the Union left.
When the fight began, the two sides clashed. The both did not know what was going to happen. The Union slowly began to get an advantage over the Confederate forces and drive them back. The Confederate forces seemed as if they were going to be defeated, but a charismatic General dug in and rallied his men to hold an important high ground at Henry House Hill.
The General, Thomas Jackson, was given the nickname Stonewall for continuing to repel Union advancements. Late in the afternoon, Confederate reinforcements arrived and extended the Confederate line and overwhelmed the Union right flank.
Colonel James Ewell Brown "Jeb" Stuart charged into a confused mass of New York infantry and drove them to the rear of the lines. In response to the heroics of Stonewall, Confederate reinforcements, and Jeb Stuart, the Union troops became confused in their retreat. Total destruction may have occurred if not for the confusion of the Confederate troops.
The battle changed the opinion of those in the North. Abraham Lincoln replaced Irwin McDowell with George McClellan and began preparations for a long and costly war.