In order to draw Pope's army into battle, Jackson ordered an attack on a Federal column that was passing across his front on the Warrenton Turnpike on August 28. The fighting at Brawner Farm lasted several hours and resulted in a stalemate. 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 Fitz John Porter's command, Longstreet's wing of 28,000 men counterattacked in the largest, simultaneous mass assault of the war. The Union left flank was crushed and the army driven back to Bull Run. Only an effective Union rearguard action prevented a replay of the First Manassas disaster. Pope's retreat to Centreville was precipitous, nonetheless. The next day, Lee ordered his army in pursuit. This was the decisive battle of the Northern Virginia Campaign.