This good fortune was the discovery of a copy of General Lee's orders of march for the army, in which his whole plan was revealed.

General McClellan had therein the unmistakable evidence of his opponent's intentions, and from that moment his advance was as rapid as before it had been deliberate.
The result of this fortunate discovery was speedily seen.

General Lee, while moving steadily toward Hagerstown, was suddenly compelled to turn his attention to the mountain-passes in his rear.

It had not been the intention of Lee to oppose the passage of the enemy through the South Mountain, as he desired to draw General McClellan as far as possible from his base, but the delay in the fall of Harper's Ferry now made this necessary.

It was essential to defend the mountain-defiles in order to insure the safety of the Confederate troops at Harper's Ferry; and Lee accordingly directed General D.H.Hill to oppose the passage of the enemy at Boonsboro Gap, and Longstreet was sent from Hagerstown to support him.
An obstinate struggle now ensued for the possession of the main South Mountain Gap, near Boonsboro, and the roar of Jackson's artillery from Harper's Ferry must have prompted the assailants to determined efforts to force the passage.