Before Antietam: The Battle for South MountainRobert E. Lee, after decisively repelling John Pope's August 1862 invasion of Virginia at the Second Battle of Manassas, took the offensive. Moving north into Maryland, Lee divided his forces to capture Harpers Ferry while continuing his advance further into Union territory. George B. McClellan, the new Union commander, learned that Lee had divided his forces, and advanced to attack the Confederates. The armies, from squad to corps level, fought hard