The force detached by Lee for this expedition consisted of a body of twelve thousand men under General Early.
Following the beaten track of invasion, Early marched rapidly down the Shenandoah Valley, arriving before Martinsburg the 3d of July.
Sigel, who held post there with a small force, at once retreated across the Potomac at Shepherdstown.
General Weber, in command at Harper's Ferry, evacuated the town and retired to Maryland Heights.
Hunter, who had made a toilsome march through the Alpine region of Western Virginia, experienced great delays in transporting his troops to Harper's Ferry, owing to the lowness of the river and the breaking of the railroad in several places.
He was therefore not in position to check the irruption of the enemy into Maryland, and the Confederates, the way being thus open, passed the Potomac, and marching by way of Hagerstown, on the 7th, reached Frederick—a central point whence they might threaten both Baltimore and Washington.
The only force at