The Second Battle of Bull Run or Battle of Second Manassas was fought August 28–30, 1862, in Prince William County, Virginia, as part of the American Civil War. It was the culmination of the Northern Virginia Campaign waged by Confederate Gen. Robert E. Lee's Army of Northern Virginia against Union Maj. Gen. John Pope's Army of Virginia, and a battle of much larger scale and numbers than the First Battle of Bull Run (or First Manassas) fought on July 21, 1861 on the same ground.

Following a wide-ranging flanking march, Confederate Maj. Gen. Thomas J. "Stonewall" Jackson captured the Union supply depot at Manassas Junction, threatening Pope's line of communications with Washington, D.C. Withdrawing a few miles to the northwest, Jackson took up strong concealed defensive positions on Stony Ridge and awaited the arrival of the wing of Lee's army commanded by Maj. Gen. James Longstreet. On August 28, 1862, Jackson attacked a Union column just east of Gainesville, at Brawner's Farm, resulting in a stalemate but successfully getting Pope's attention. On that same day, Longstreet broke through light Union resistance in the Battle of Thoroughfare Gap and approached the battlefield.

Pope became convinced that he had trapped Jackson and concentrated the bulk of his army against him. On August 29, Pope launched a series of assaults against Jackson's position along an unfinished railroad grade. The attacks were repulsed with heavy casualties on both sides. At noon, Longstreet arrived on the field from Thoroughfare Gap and took position on Jackson's right flank. On August 30, Pope renewed his attacks, seemingly unaware that Longstreet was on the field. When massed Confederate artillery devastated a Union assault by Maj. Gen. Fitz John Porter's V Corps, Longstreet's wing of 25,000 men in five divisions counterattacked in the largest simultaneous mass assault of the war. The Union left flank was crushed and the army was driven back to Bull Run. Only an effective Union rear guard action prevented a replay of the First Manassas defeat. Pope's retreat to Centreville was nonetheless precipitous.

Success in this battle emboldened Lee to initiate the ensuing Maryland Campaign, the South's invasion of the North.