On September 2nd, 1862, the Maryland Campaign officially began with the first cavalry skirmish at Mile Hill north of Leesburg. Over the next few days, Confederate divisions of Lee's Army of Northern Virginia started making their way into the town. From September 4th - 6th, the army crosses the Potomac River at White's Ford, entering Maryland to begin Lee's first invasion of the North. Lee hopes that McClellan's overcautious nature will give him at least a week's head start on the Union Army of the Potomac still in Washington. Still, by the 4th, McClellan had reorganized the army and set it on the march to meet Lee's forces in Western Maryland. Lee's troops occupy Frederick, where the General plans to deal with the Union garrisons at Harpers Ferry and Martinsburg, labeled Special Order No. 191. This document, now famous as the "Lost Order," will come into the hands of General McClellan, who will famously remark, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home." But will McClellan's information coup be enough for the General to defeat Lee and his Confederates as they march out of Frederick through the passes of South Mountain?