This skirmish took place in Maryland. While most of Lee's army was attacking Harpers Ferry, the rest moved into Maryland with Major General George B. McClellan's Army of the Potomac closely behind them. At Frederick, Maryland the Union army caught up with Lee and the fighting began on South Mountain. It was a natural formation that separated the Shenandoah and Cumberland Valleys from eastern Maryland. Several battles were fought that day over possession of South Mountain. The Confederates were driven back with a large number of casualties. McClellan could have destroyed Lee's army but he waited giving Lee the time to regroup. 