The History: During Robert E. Lee's first invasion of the North, Lee learned of a threatening Federal advance East of Hagerstown, Maryland. Because Stonewall Jackson was besieging Harper' Ferry, he instructed Daniel Harvey Hill to hold the South Mountain passes in the rugged Blue Ridge, just East of Boonsboro. It was essential to delay McClellan's progress so Jackson would have time to take Harper's Ferry and reunite his scattered divisions before the Army of the Potomac caught him unprepared.
Despite strong defensive features the numerically superior Federal forces should have overwhelmed the rebels but the inaction of Pleasonton and Burnside squandered their advantage. In the end Lee's defense of the South Mountain passes cost him 2800 casualties as opposed to the Federal losses of 2300 and lead directly to the fearful slaughter at Antietem.