Lee observed that Pope's position was weak at Culpeper and determined to attack him without delay and gave orders for his army to cross the Rapidan. Pope knew that his position at Culpeper was weak and fell back to a stronger position behind the Rappahannock.
Lee hoped to attack the Army of Virginia before it could be reënforced by McClellan, but, on account of heavy rains, which raised the streams, he was somewhat delayed until Pope had been reënforced by a part of Burnside's corps, under General Reno, and later was also reënforced by Generals Kearny and Reynolds with their divisions of the Army of the Potomac.
Lee sent the dauntless cavalry leader J. E. B. Stuart to make a raid around the Union army. Stuart crossed the Rappahannock with 1,500 mounted men, as bold as himself. After riding all day, and on the night of the 22d, in the midst of a torrential rainstorm, while the darkness was so intense that every man was guided by the tread of his brother horseman, Stuart fell upon the Federals at Catlett's Station, capturing 200 prisoners and scattering the remaining troops in the darkness. He seized Pope's dispatch-book, with his plans and private papers, took several hundred horses and destroyed a large number of wagons loaded with supplies. Among his trophies was a fine uniform cloak and hat, which were the personal belongings of General Pope. These were exchanged later for General Stuart's plumed hat, which he had left behind when surprised by a party of Federals.
Stuart's raid proved a serious misfortune for Pope's army. But Lee had far greater things in store. He resolved to send Jackson to Pope's rear with a large force, Jackson led his army westward, which was shielded by woods and low hills of the Blue Ridge. He passed through a quiet rural community. The majority of the country folk had never seen an army before, though it is true that for many days they had heard the roar of the cannon from the valley of the Rapidan.
General Lee, in the meantime, had kept Longstreet in front of Pope's army to make daily demonstrations, to divert Pope's attention from Jackson's movements and lead him to believe that he was to be attacked in front.
Jackson suddenly, on August 26th, emerged from the Bull Run Mountains and marshaled his clans on the plains of Manassas.
Pope was astonished to find Jackson in his rear, and hastened with all speed with his forces toward Manassas Junction, where he had vast stores of provisions and munitions of war, but he was too late to save them. They had been taken by General Stuart in advance of Jackson's army. This was a serious loss to Pope. The spoils of the capture were great, including 300 prisoners, 125 horses, ten locomotives, seven long trains of provisions, and vast stores and munitions of war. Pope was moving against Jackson with a far larger army, and was expecting to be reënforced from the Army of the Potomac, while on the other hand, Longstreet was hastening to reënforce Jackson, but had not arrived.
Pope, hoping to crush Jackson's army before he could be reënforced by Longstreet, sent a force to interpose Longstreet at Thoughfare Gap. Jackson was not to be caught in a trap. He moved from Manassas Junction to the old battlefield of Bull Run.
Late in the afternoon of the 29th he encountered King's division of McDowell's corps, near the village of Groveton, and a sharp fight was opened and kept up until after dark.
On the following day, August 29th, the first day's battle was fought. Pope was still hopeful of crushing Jackson's army before the arrival of Longstreet, and ordered a general advance across Bull Run.