On July 21, 1861, 35,000 Union troops led by Brigadier General Irvin McDowell faced off against more than 20,000 Confederates under Brigadier General P.G.T. Beauregard near a railroad junction at Manassas, Virginia, 25 miles from Washington, D.C. The First Battle of Bull Run, also known as Manassas, was the first major land battle of the American Civil War. To commemorate the battle’s 150th anniversary, we asked HISTORY’s chief historian Libby O’Connell about its significance and what every American can do to help preserve our shared history.

What events led up to the First Battle of Bull Run?

The war had begun three months earlier at Fort Sumter, but since then there had only been small-scale clashes between the two sides. Abraham Lincoln decided to strike first. The Union strategy was to deal a crushing blow to Confederate forces near Manassas, Virginia, and quickly march on Richmond, the Confederate capital. Union General Irvin McDowell, worried that his untrained troops were unprepared for such an endeavor, protested the plan, but Lincoln overruled him. Why did he do this? Well, there was enormous pressure in the North, primarily from the press, for a quick, decisive action to end the war. More importantly, however, the 90-day term of enlistment for most of the soldiers who had joined the Union Army after Fort Sumter was set to expire. Lincoln believed that this might be his only chance to use this massive military force before he lost it.

What exactly happened during the battle?

At first, the Union Army seemed on the verge of victory, pushing back P.G.T. Beauregard’s forces at Warrenton turnpike, while one Southern brigade, led by General Thomas “Stonewall” Jackson, desperately tried to hold the high ground at Henry Hill House. The tide of the battle turned later in the day, with the arrival of Confederate reinforcements under the command of Joseph E. Johnston. The increasingly disorganized and overwhelmed Union Army collapsed under the pressure of the Confederate counterattack, resulting in a frantic retreat. However, the Confederate troops were just as exhausted and disorganized, and they were unable to mount a pursuit of the fleeing Union Army.

What happened in the aftermath of the battle?

It took the shattered Union Army nearly 36 hours to get back to Washington, D.C., marching almost without rest or food. As one soldier put it, this army that was supposed to crush the Confederates limped back into the capital “more dead than alive.”...