Special Order 191 was the Army of Northern Virginia’s plan for the Maryland Campaign. It fell into the hands of General McClellan and gave him the full outline of what Lee planned to do.

To paraphrase Special Order 191,its 10 parts were as follows:
I No soldier was to go into Frederick, MD without a written pass.
II Major Taylor will go to Leesburg and provide transportation of sick and wounded to Winchester, VA.
III General Jackson after passing Middleton, MD will proceed to Sharpsburg, MD; cross the Potomac River; and take Martinsburg, VA.
IV General Longstreet will proceed to Boonsborough, MD and halt there.
V General McLaw’s Division and General Richard Anderson’s Division will follow General Longstreet and on reaching Middleton will take the route to Harper’s Ferry and by Friday, September 12th, take Maryland Heights.
VI General Walker will continue with his present project; cross the Potomac River at Cheek’s Ford; ascend the right bank to Lovettsville, VA; take Loudoun Heights by Friday the 12th of September.
VII General Daniel Hill will form rear guard of all artillery, ordinance and supply trains and precede him.
VIII General Stuart will detach cavalry to accompany all Longstreet’s, Jackson’s and McLaw’s Commands and bring up stragglers.
IX Commands of Jackson, McLaw and Walker after accomplishing their objectives will join the main body at Boonsborough, MD or Hagerstown, MD.
X Each regiment will carry axes in the regimental wagons.

Walker’s Division had been assigned a major part in the capture of Harper’s Ferry.

Walker on the 10th proceeded east toward the aqueduct and Cheek’s Ford, which was just to the east of the aqueduct. He wanted to cross the Potomac at Cheek’s Ford, but saw that Union Forces occupied the aqueduct and had artillery support.

Walker then proceeded west and during the night of the 10th and early morning of the 11th crossed the Potomac at Point of Rocks. The division then went into bivouac on the 11th.

What happens next has been the basis of much speculation.