The first major battle of the American Civil War occurred on July 21, 1861, in Manassas, Virginia. The Battle is known both as the First Battle of Bull Run, after the creek that ran through the battlefield, or, the First Battle of Manassas. Union forces under General Irvin McDowell, hoping for a decisive blow against the newly formed Confederacy, planned a surprise attack against the left flank of Confederate General P.G.T. Beauregard's forces at the railroad junction of Manassas. Both sides were unseasoned and unprepared for the carnage that would ensue. Hundreds of spectators from Washington lined the hills of the battlefield to cheer on the Union. McDowell, who was skeptical of the Union plan, amassed 35,000 soldiers - the largest land army ever assembled in America at the time, to attack the Confederate positions.

At 2:30 in the morning on July 21, McDowell sent two detachments from nearby Centreville toward Confederate positions. By 5:15, the first shots had been fired. Through the morning, Union forces had slowly pushed Confederate forces back until they broke through the Confederate right flank at about 11:30, sending them into a disorganized retreat. Gen McDowell elected not to pursue the retreating Confederates, who were able to regroup at the Henry Hill House. At 12:00, Confederate reinforcements under General Thomas J. Jackson arrived on the battlefield. It was at this moment, when Confederate General Bernard Bee is said to have uttered the timeless words "There is Jackson standing like a stone wall." From that point on Thomas J. Jackson would be known as Stonewall Jackson. Some historians believe Bee was actually angry at Jackson for not sending his troops directly into battle. The tide of the battle soon changed when Confederate forces under Jackson and Calvary Commander J.E.B. Stuart captured Union artillery around 3:00. Jackson commanded his soldiers to press on and for the first time in the war, Union forces would be horrified by the "rebel yells" screamed by Confederate soldiers on the attack. At 4:00, more Confederate reinforcements led by Kirby Smith and Jubal Early crushed a Union brigade and caused the collapse of McDowell's Union Army. Union forces began a disorderly and poorly managed retreat, accompanied by screaming spectators who clogged the retreat path. Confederate forces, also in a state of disarray, captured several hundred Union soldiers but failed to pursue the remaining forces back to Washington.

The First Battle of Bull Run proved to be the deadliest battle in American history to that point. Nearly 5,000 total casualties were recorded, with Union forces suffering roughly 1,000 more than the Confederates. The North was shocked and dismayed at the Union defeat and many feared the Confederacy would move immediately on Washington. At the very least, the First Battle of Bull Run proved the war would be long and violent. President Lincoln was forced to sign a bill that allowed for the enlistment of up to 500,000 additional soldiers.