If the enemy had succeeded in posting forces in the gaps of the Bull Run range of mountains, and in occupying the wooded country between Thoroughfare Gap and Leesburg, they would not only have hidden all their own movements from view, but would have had command of the Potomac from Harper's Ferry to within thirty miles of Washington, so that they could have operated on either side of the river.
While Gregg's division were thus engaged, Colonel Duffie started under orders with his regiment from Centreville for Middleburg, by way of Thoroughfare Gap.

The enemy (W.H.F.Lee's brigade) were already there, but he forced them out, and kept on to Middleburg, which was reached about 9.30 A.M.

He found Stuart's rear guard or escort there, and drove them out.

Stuart fell back to Rector's Cross Roads, and sent word to all his forces to concentrate against Duffie.

Duffie barricaded the streets of the town and prepared to hold it until reinforcements could reach him from Aldie, not being aware that there was any impediment in that direction.