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.