The resounding Confederate victory at the Battle of Manassas/Bull Run (Second) spurred Gen. Robert E. Lee to renew his offensive and destroy the demoralized Union Army of Virginia.
On August 30, Maj. Gen. John Pope ordered his Army of Virginia to retreat about 5 miles to the northeast to Centreville. The movement began after dark, with Brig. Gen. Irvin McDowell's III Corps providing cover. The army crossed Bull Run Creek and the last troops across destroyed Stone Bridge behind them. Lee also allowed the II Corps of Brig. Gen. Nathaniel P. Banks to consolidate with the remainder of Pope's army, marching in from Bristoe Station, where they had been guarding the army's trains.
On the day after the battle, Lee sent Lt. Gen. Thomas J. "Stonewall" Jackson on a flanking movement to get behind Pope's position on the heights near Centreville. Lt. Gen. James Longstreet's corps remained in place for the day to deceive Pope while Jackson's corps made a wide flanking march, north and then east, attempting to block the Warrenton Turnpike and Pope's escape route. The corps moved slowly over muddy roads and bivouacked for the night at Pleasant Valley, 3 miles northeast of Centreville. Pope anticipated this maneuver and telegraphed Halleck that he could deal with it. If Lee's plan succeeded, the Union army might be struck a fatal blow before they could find shelter in the defenses surrounding Washington, D.C.
Jackson's route on August 31 took him across Bull Run at Sudley Ford, then to the Little River Turnpike and down that road toward Fairfax Court House. A nightime rain turned the roads into troughs of mud, and the Confederates made little progress. By nightfall, his veterans were slogging into Pleasant Valley. With their wagon train far to the rear, the Confederates went to sleep without food.
On September 1, Jackson's hungry and exhausted troops crawled eastward on the turnpike on the morning. Dark, lowering clouds portended more rains as the Confederates reached the country mansion of Chantilly. Early in the afternoon, the Confederates met the Union army, who were deployed in numbers and ready. Pope's cavalry had detected Lee's flank movement, and Pope recalled some retreating units to meet the threat.
Jackson's troops marched only 3 miles and occupied Ox Hill, southeast of Chantilly Plantation, where they encountered the Federals at about 3:00 P.M. Jackson deployed his 3 divisions with his artillery across the road. Maj. Gen. A.P. Hill, on the right, sent in 2 brigades. A thunderstorm suddenly broke over the fields, its winds driving sheets of rain into the faces of the charging Confederates. The 2 brigades crossed a field and entered a stand of woods. Two Union divisions, under Maj. Gen. Philip Kearny and Brig. Gen. Issac I. Stevens, responded with a volley. The 2 Confederate brigades clung to their woodline position as the combat intensified.
Hill threw another brigade forward, then 2 more. The torrential downpour added to the confusion and misery. Jackson then commited his left division, commanded by Brig. Gen. William E. Starke, but the Federals repulsed this attack directly south of the turnpike. The fighting had twisted Jackson's lines into an arc of a circle facing southward.
Attacks and counterattacks flared across the soaked woodlands and fields. Neither side gained much of an advantage until Stevens was killed, and Kearny fell dead after riding unaware into the midst of some Confederate skirmishers. Despite having numerical superiority over the Union defenders, the attacks were repulsed until they petered out by 6:30 P.M. Recognizing that his army was still in danger at Fairfax Courthouse, Pope ordered the retreat to continue to Washington in the gathering darkness.
Kearny, one the Union's most aggressive and respected generals, mistakenly rode into the Confederate lines during the battle and was killed. Stevens was also killed on his front, while bravely waving the colors to rally his men. That night, Longstreet arrived to relieve Jackson's troops. The Union army withdrew to Germantown and Fairfax Court House.
With Pope no longer a threat, Lee turned his army west and north to invade Maryland, initiating the Maryland Campaign and the battles of South Mountain and Antietam. Maj. Gen. George B. McClellan assumed command of Union forces around Washington.
The fighting was tactically inconclusive, but Jackson's turning movement was foiled and he was unable to block the Union retreat or destroy Pope's army. Pope, recognizing the attack as an indication of continued danger to his army, continued his retreat to the fortifications around Washington, D.C. Lee began the Maryland Campaign after Pope retreated out of Virginia. The driving rainstorm probably saved Jackson from a serious defeat, for the weather delayed Pope from sending into battle the 2 nearby Union corps. Nevertheless, Pope's men held, thwarting Lee's offensive.
The Army of the Potomac, under Maj. Gen. George B. McClellan, absorbed the forces of Pope's Army of Virginia, which was disbanded as a separate army.
The Battle of Chantilly, also known as the Battle of Ox Hill, took place as the concluding battle of the Second Manassas/Bull Run Campaign.
Neither side gained any advantage worth mentioning. During part of the time it rained heavily, and it grew very dark before the action closed. Both sides lost severely. The Federal reports are either altogether lacking or very meagre, but we learn from the Confederate reports that this was an unsatisfactory fight for Jackson's Corps. Branch's brigade of Hill's division was thrown into great disorder by a flanking fire, and its commander, General Lane, says the engagement was considered by the brigade as one of the severest. Gregg's brigade, that lost so many men at Manassas, here again suffered heavily. Trimble's brigade evidently had a severe experience, and all that is claimed is that they held their position, which they certainly did.
On the Federal side, we lost in General Stevens a resolute, clear-headed, able officer. Kearny was a man made for the profession of arms. In the field he was always ready, always skilful, always brave, always untiring, always hopeful, and always vigilant and alert.
These severe losses and the indecisive character of the engagement, which after all was only a repulse of the enemy, could not restore the morale of the army. The enemy pursued his design of outflanking our right. Longstreet was up in the course of the night. On September 2d, at noon, the army being weary and the Government evidently subjected to great pressure, the order was given to withdraw the troops within the lines of Washington, and the campaign of the army under Pope was ended. Source: Chapter 11 of "The Army Under Pope" By John Codman Ropes