The Second Battle of Bull Run occurred between August 28 and August 30, 1862. President Abraham Lincoln, dismayed by the performance of Union Commander George B. McClellan, placed John Pope in command of the newly formed Army of Virginia. Pope's Army of Virginia consisted of three corps totaling about 77,000 men. Their purpose was to protect Washington and the Shenandoah Valley and to ensure that Confederate forces would be diverted away from McClellan's Army by marching in the direction of Gordonsville, Virginia.

Robert E. Lee, realizing that two main bodies of the Union army were split and out of range of each other saw an opportunity to crush Pope's army. He first sent the armies of Generals Stonewall Jackson and A.P. Hill directly to Pope's supply line - the Orange and Alexandria Railroad where Jackson oversaw its destruction before marching into Manassas and capturing and destroying the Union supply depot there. This shocking development forced Pope to move his troops from their defensive positions and caused Jackson to take defensive positions near an unfinished railroad grade close to the original Bull Run battlefield. The Second Battle of Bull Run started on August 28, when Jackson attacked a Union column marching along the Warrenton Pike near Gainesville, Virginia. Neither side had earned the advantage on the 28th, but fierce fighting at close range resulted in nearly 2,400 total casualties. Two Confederate generals, including Richard Ewell, were badly injured.

Pope saw the outcome of the first day as a Union victory and believed Jackson was retreating. Consequently, he ordered an assault on Jackson for the 29th, whose army had taken up effective defensive positions and, who, was soon to be reinforced by Confederate General James Longstreet and an additional 25,000 soldiers. Confusion in the Union ranks led to severe miscommunication and faulty information concerning the arrival of Longstreet. Furthermore, Pope's written orders to his generals were vague and contradictory, further frustrating mobilization. Various assaults and battles occurred through the day and night of the 29th nevertheless. By the end of the night, Pope still believed the Confederates to be in a state of retreat and ordered a massive offensive for the 30th.

On the morning of the 30th, Confederate forces were in defensive positions, hoping to be attacked so they could launch a counterattack. Pope launched his massive offensive into Jackson's Army shortly after noon. Confederate artillery devastated Union forces under Fitz John Porter before launching a counterattack with Longstreets 25,000 soldiers that proved to be the largest mass assault in the war, utterly crushing the entire left flank of Union forces and sending them back to Bull Run Creek and ultimately along the Warrenton Turnpike to Centreville. Because of exhaustion and darkness, the Confederates did not pursue, and although Pope had lost the battle, his army was not totally destroyed.

The Second (and much larger) Battle of Bull Run was a decisive Confederate victory. The Union suffered over 10,000 casualties, while the Confederacy suffered over 7,000. The Confederate victory allowed Robert E. Lee to plan and execute his Maryland campaign. Pope's performance in the battle cost him his command. Union Major General Fitz John Porter was court-martialed for failing to follow Pope’s orders and was dismissed from the Army.