Both sides assumed that war would be short-lived. President Lincoln had called for 90 day volunteers after the Confederates had opened cannon fire on the Union troops inside Fort Sumter, a coastal fort near Charleston, South Carolina in April.
With these troops' enlistments moving toward expiration the Union commander, Brigadier General Irvin McDowell felt pressure to make some kind of move toward the Confederate capitol, Richmond, Virginia. His 35,000 or so ill-trained soldiers met around 30,000 similarly trained troops commanded by two southern generals, Pierre Gustave Toutant Beauregard, Confederate hero of Ft Sumter and Joseph E. Johnston of Virginia.
The two generals laid plans which would have had the armies dance around one another, each planning to attack the others right side or flank (in military terms.) As it happened the Union soldiers moved more quickly and forced the Confederates to retreat from Bull Run, until one brigade (around 3000 soldiers) led by Thomas J Jackson, a teacher, held fast. Other confederate leaders told their men that this group were standing like stone walls and the most famous nickname of the civil war was born.
Confederate reinforcements now arrived and pushed the Union soldiers back. This movement became a retreat and then a rout with Confederate cavalry trying to pursue fleeing soldiers.
Many of the Union soldiers had a hard time escaping quickly because the roads from Manassas back to Washington were now clogged with the carriages of Washingtonians who had come to the battlefield to view the spectacle. That war could be dangerous and deadly had apparently not yet occurred to them, and that was one of many casualties on the 21st of July, 1861.
The photo below was taken in March, 1862 (8 months after the battle) by George Barnard and shows Bull Run and hills which were important in the battle.
