On September 9th Lee had written Special Order 191, which explained his plan for the campaign. He ordered three columns under the overall direction of Stonewall Jackson to converge on Harper's Ferry and capture it. This would open the Shenandoah Valley and eliminate a force in the Confederate rear. At that point Lee could move north into Pennsylvania with his rear secure. However, it would be a dangerous maneuver, splitting his army into several separate columns while McClellan was still moving to pursue. The order said this:
Special Orders, No. 191
Hdqrs. Army of Northern Virginia
September 9, 1862
… The army will resume its march tomorrow, taking the Hagerstown road. General Jackson's command will form the advance, and, after passing Middletown, with such portion as he may select, take the route toward Sharpsburg, cross the Potomac at the most convenient point, and by Friday morning take possession of the Baltimore and Ohio Railroad, capture such of them as may be at Martinsburg, and intercept such as may attempt to escape from Harpers Ferry.
General Longstreet's command will pursue the same road as far as Boonsborough, where it will halt, with reserve, supply, and baggage trains of the army.
General McLaws, with his own division and that of General R. H. Anderson, will follow General Longstreet. On reaching Middletown will take the route to Harpers Ferry, and by Friday morning possess himself of the Maryland Heights and endeavor to capture the enemy at Harpers Ferry and vicinity.
General Walker, with his division, after accomplishing the object in which he is now engaged, will cross the Potomac at Cheek's Ford, ascend its right bank to Lovettsville, take possession of Loudoun Heights, if practicable, by Friday morning, Key's Ford on his left, and the road between the end of the mountain and the Potomac on his right. He will, as far as practicable, cooperate with General McLaws and Jackson, and intercept retreat of the enemy.
General D. H. Hill's division will form the rear guard of the army, pursuing the road taken by the main body. The reserve artillery, ordnance, and supply trains, &c., will precede General Hill.
General Stuart will detach a squadron of cavalry to accompany the commands of Generals Longstreet, Jackson, and McLaws, and, with the main body of the cavalry, will cover the route of the army, bringing up all stragglers that may have been left behind.
The commands of Generals Jackson, McLaws, and Walker, after accomplishing the objects for which they have been detached, will join the main body of the army at Boonsborough or Hagerstown.
Each regiment on the march will habitually carry its axes in the regimental ordnance—wagons, for use of the men at their encampments, to procure wood &c.
By command of General R. E. Lee
R.H. Chilton, Assistant Adjutant General
McClellan was overcome with joy when he read the order. It gave him the Confederate plans, and now he only had to push over South Mountain and then fall upon the scattered Confederate columns and destroy them. He told an officer, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home." this paper was the key to the campaign. The providential loss of the order would result in the failure of Lee's plans for invasion, instead turning the campaign into a desperate effort to save his army.