By September 13th McClellan’s forces had reached Frederick Md. and by happenstance discovered dispatch 191, or what has become known as the “Lost Dispatch” or the “Lost Order.”

Lee had drawn up the orders on September 9, 1862. The details of his explicit plans for the travels of the Army of Northern Virginia during the foray into of Maryland. Lee’s plan separated his army into several sectors, which he intended to reorganize later: Lee sent Maj. Gen. Stonewall Jackson’s troops to Harpers Ferry with the initiative of capturing the Union barracks and provisions there. The dispatch ordered Maj. Gen. James Longstreet north to Boonsboro, MD and the major group of the force to Hagerstown, MD

This dispatch was in essence the battle plan, which Lee had drawn for the invasion of Maryland. By McClellan recovering this information, the intelligence had a great impact on the outcome of the campaign that ensued with the battles of South Mountain and Antietam both. On September 14: The Union’s First and Ninth Corps capture Turner’s Gap from Hill and Longstreet's forces, compelling a Confederate withdraw.

Between the 13th and 15th Stonewall Jackson’ troops had sealed off the southern most escape route from Harpers Ferry and begins shelling Harpers Ferry’s garrison on the 15th. This portion of the incursion was successful and some 12,000 Union forces surrendered at Harper’s Ferry on the 15th. With the fall of the Harpers Ferry’s garrison, Lee decides to make as strong stand at the Antietam Creek near Sharpsburg MD.

Next, the Battle ensues.