Lee delineated the routes and roads to be taken and the timing for the investment of Harpers Ferry. Adjutant Robert H. Chilton penned copies of the letter and endorsed them in Lee's name. Staff officers distributed the copies to various Confederate generals. Jackson in turn copied the document for one of his subordinates, Maj. Gen. Daniel Harvey Hill, who was to exercise independent command as the rear guard.Hill said the only copy he received was the one from Jackson.
About 10 a.m. on September 13, 1862, Corp. Barton W. Mitchell of the 27th Indiana Volunteers, part of the Union XII Corps, discovered an envelope with three cigars wrapped in a piece of paper lying in the grass at a campground that Hill had just vacated. Mitchell realized the significance of the document and turned it in to Sgt. John M. Bloss. They went to Capt. Peter Kopp, who sent it to regimental commander Col. Silas Colgrove, who carried it to the corps headquarters. There, an aide to Brig. Gen. Alpheus S. Williams recognized the signature of R.H. Chilton, the assistant adjutant general who had signed the order. Williams forwarded the dispatch to Maj. Gen. George B. McClellan, the commander of the Army of the Potomac. McClellan was overcome with glee at learning planned Confederate troop movements and reportedly exclaimed, "Now I know what to do!" He confided to a subordinate, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home."
McClellan stopped Lee's invasion at the subsequent Battle of Antietam, but many military historians believe he failed to fully exploit the strategic advantage of the intelligence because he was concerned about a possible trap (posited by Maj. Gen. Henry Wager Halleck) or gross overestimation of the strength of Lee's army.
The hill on the Best farm where the lost order was discovered is located outside of Frederick, Maryland, and was a key Confederate artillery position in the 1864 Battle of Monocacy. A historical marker on the Monocacy National Battlefield commemorates the finding of Special Order 191 during the Maryland Campaign.