"Today will be known as BLACK MONDAY. We are utterly and disgracefully routed, beaten, whipped by secessionists." - George Templeton Strong, Northern diarist

After the Battle of Fort Sumter in April 1861 ignited the Civil War, many in the North expected a relatively quick victory, including President Abraham Lincoln. While that seems naïve in hindsight, given the knowledge that the war lasted over four years, these expectations seemed entirely realistic at the time due to the Union's overwhelming economic advantages over the South. At the start of the war, the Union had a population of over 22 million, whereas the South had a population of 9 million, nearly 4 million of whom were slaves. Union states contained 90% of the manufacturing capacity of the country and 97% of the weapon manufacturing capacity. Union states also possessed over 70% of the total railroads in the pre-war United States at the start of the war, and the Union also controlled 80% of the shipbuilding capacity of the pre-war United States.

After Fort Sumter, the Lincoln Administration pushed for a quick invasion of Virginia, with the intent of defeating Confederate forces and marching toward the Confederate capitol of Richmond. Lincoln pressed General Irvin McDowell to push forward. Despite the fact that McDowell knew his troops were inexperienced and unready, pressure from Washington politicians forced him to launch a premature offensive against Confederate forces in Northern Virginia.

McDowell's strategy during the First Battle of Bull Run was grand, and in many ways it was the forerunner of a tactic Robert E. Lee, Stonewall Jackson and James Longstreet executed brilliantly on nearly the same field during the Second Battle of Bull Run in August 1862. McDowell's plan called for parts of his army to pin down General P.G.T. Beauregard's Confederate soldiers in front while marching another wing of his army around the flank and into the enemy's rear, rolling up the line. McDowell assumed the Confederates would be forced to abandon Manassas Junction and fall back to the next defensible line, the Rappahannock River. In July 1861, however, this proved far too difficult for his inexperienced troops to carry out effectively.

As the first major land battle of the Civil War, the First Battle of Bull Run made history in several ways. The two sides fought near the railroad junction at Manassas on July 21, 1861, just 25 miles away from Washington, which was close enough for many civilians from the nation's capital to come and watch what they expected to be a rout of Confederate forces, and for awhile it appeared as though that might be the case.

McDowell's strategy ultimately fell apart though, mostly thanks to the use of the railroad. Confederate reinforcements under General Joseph E. Johnston, including a brigade led by Thomas Jonathan Jackson, arrived by train in the middle of the day, a first in the history of American warfare. With Johnston's army arriving midday on July 21, it evened up the numbers between the Union and Confederate forces, and by shoring up the Confederates' left flank, some of Johnston's troops, most notably Jackson's brigade, helped reverse the Union's momentum and ultimately turn the tide.

As the battle's momentum switched, the inexperienced Union troops were routed and retreated in disorder back toward Washington in an unorganized mass. With over 350 killed on each side, it was the deadliest battle in American history to date, and both the Confederacy and the Union were quickly served notice that the war would be much costlier than either side initially anticipated.