Casualties at the Battle of First Bull Run (Battle of First Manassas), the first major engagement of the Civil War, were light in comparison with battles that would come later. Union brigadier general Irwin McDowell marched approximately 28,400 men in the Army of Northeastern Virginia from Washington, D.C., to confront roughly 21,900 Confederates of the Army of the Potomac under Brigadier General P.G.T. Beauregard. During the afternoon, some 8,900 men of General Joseph Johnston’s command arrived to reinforce Beauregard, resulting in a Union rout.
Union Casualties at First Bull Run (approximate)
McDowell’s 28,400 men suffered 480 killed, 1,000 wounded, and 1,200 missing, for a total loss of 2,680 casualties, approximately 9.5 percent.
Confederate Casualties at First Bull Run (approximate)
Beauregard and Johnston’s combined force of 30,800 had 390 killed, 1,600 wounded, and about a dozen missing, a total of approximately 2,000 or about 6.5 percent.
Both sides suffered about the same number of killed and wounded. Surprisingly, the Confederates, who were on the defensive, lost the most in these categories; usually, the side that is attacking sustains the most killed and wounded. The Union’s greatest losses were in the "missing" category, where they sustained 100 times the number that the victorious Confederates did. Some of the men who routed decided they’d had enough of soldiering, but many Federal soldiers were taken prisoner and sent to Richmond. From Richmond, a number of them were sent on to cities farther south, such as Salisbury, North Carolina. At this stage of the war, both sided practiced prisoner exchanges, allowing many of the captured to eventually return to the ranks.