It was time for McClellan to do something.
Frederick, the town in Maryland where McClellan’s headquarters was located, was about 40 miles northwest of Washington. The National Road came up from Washington, passed through Frederick, and then continued northwest towards Hagerstown for about 25 miles. From there, there are good roads dropping south into Virginia and the Shenandoah, other roads lead west into Ohio, and some go into Pennsylvania. About halfway between Frederick and Hagerstown lays South Mountain. It is not a mountain, per se, but rather a long, curving ridge that starts by the Potomac next to Harper’s Ferry and runs far into Pennsylvania and passes west of Gettysburg. From McClellan’s point of view, South Mountain was on the horizon – a dark, long curtain that separated the Union army and the Confederates. It was a veil on the far side of which lay the full power of the Confederacy, fully shielded from the prying eyes of the Federals.