...nth McClellan had reached Frederick, while Lee was by that time across the Catoctin range at Boonsboroa, but his army was divided. He had sent a large part of it back across the Potomac to capture Harper's Ferry and Martinsburg. On that day there fell into McClellan's hands the copy of an order issued by General Lee three days before, which, as McClellan himself states in his report, fully disclosed Lee's ...