General Jackson withdrew his forces after the battle towards Gordonsville, to wait the advance of the main army, under Lee, while General Pope pushed south to the Rapidan.
On the 16th, General Pope's cavalry captured a Rebel courier, who was bearing a letter from Lee to Jackson, from which it was ascertained that the whole of Lee's army was moving north from Richmond, to crush Pope before McClellan could join him. General Pope was prompt to act upon this information. He retreated to the north bank of the Rappahannock, planted his artillery to cover the fords, hoping to hold Lee in check till he was reinforced.
Lee followed rapidly with his whole army. He reached the Rappahannock on the 21st, attempted to cross, but was foiled in all his movements.
Suddenly, on the night of the 22d, General Stuart fell upon the Orange and Alexandria Railroad at Catlett's Station, in General Pope's rear. It was a dark, rainy night. Many army wagons were there, and some were burned. All the horses were taken. General Pope lost his personal baggage.
In the morning, General Pope understood that it was Lee's intention to gain his rear, and cut him off from Washington. Jackson was moving along the base of the Blue Ridge by swift marches.
The mountains, which at Leesburg are called the Catoctin Range, farther south are called the Bull Run Mountains. There is a gap at Aldie, and another one at the head of Broad Run, called Thoroughfare Gap. There the mountain is cut down sharp and square. There is room for the railroad, the turnpike, and the creek. A hundred men might hold it against a thousand. That part of the mountain south of the gap is about ten miles long.
One day I climbed the ridge to take a look at the surrounding country. Northward I could see the gap. A mile or two east of it, on the Manassas Gap Railroad, was the little village of Gainsville. Directly east was the cluster of houses called Greenwich, on the Warrenton and Centreville Turnpike. Ten miles distant, a little south of east, was Manassas Junction. Bristow's Station is south; Catlett's, southwest. Warrenton, one of the prettiest towns in Virginia, lies at the foot of the mountain, southwest, with roads radiating in all directions, as if it were the body of a spider, and the highways were legs. Westward is the Blue Ridge, looming dark and high, like an ocean billow ready to break over all the surrounding plains. In the northwest are the Cobble Mountains,—hillocks which lie between Bull Run and the Blue Ridge. Upon the railroad which winds towards Manassas Gap is the town of Salem.
If I had stood there on the 26th of August, I should have seen a body of Rebel troops moving across from the base of the Blue Ridge, through fields, through forests, and along the highways, towards Salem with great rapidity,—the men footsore, weary,—many of them barefoot, few of them decently dressed,—but urged on by their officers. It is Jackson's corps pushing for Thoroughfare Gap.
At Warrenton, General McDowell is breaking camp, and moving east over the Centreville turnpike to reach Gainsville. General Sigel follows him. General Reno, with Burnside's troops, is marching for Greenwich. General Kearny's and General Hooker's men, who have fought at Williamsburg, Seven Pines, Glendale, and Malvern, have joined Pope, and are moving along the Orange and Alexandria Railroad. General Porter is at Warrenton Junction. General Banks is coming up near the Rappahannock to join Porter.