Union General McDowell was wary about this sort of action, as his men were inexperienced and green, just like the Confederate forces. Intense political pressure eventually pushed McDowell to march into action, against his better judgement. He set out with a large force of 35,000 men, hoping to send two columnns to attack the Confederate line of Bull Run to occupy their attention. His plan was to then send the third columnn to the rear of the Confederate line, cutting off their supply line to Richmond. McDowell's plan was designed to force a Confederate retreat and relieve some pressure on the Capital.
This plan, however, was poorly executed by McDowell's subordinate officers; they took too long positioning themselves. The surprise flank attack was nonetheless effective, putting the Confederate forces into dire straits. The time it took the Union troops to position themselves became troublesome, as it allowed Confederate reinforcements to arrive by rail. These reinforcements were fresh. They did not have to march a great distance to the battlefield and had not been involved in the pitched combat.
The arrival of Confederate General Johnston from the Shenandoah Valley quickly turned the tide of the battle. It was at this point Stonewall Jackson and his famous brigade held the line (it was here he earned his nickname). Morever, during Stonewall's attack on Henry House Hill, the charging Confederates led out terrifying high pitched battle cries, which later became known as the rebel yell.
The Confederate counterattack was strong and forced a Union retreat. Due to the inexperience of the soldiers, however, this retreat quickly turned into a disastrous and disorganized rout. The resulting bloody Confederate victory was a clear sign to many that the war would not end quickly, as was popular notion.