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[[File:gaps.jpg|thumbnail|250px|left|Troop movement in and around the Gaps.]]The arsenal and supply depot at Harpers Ferry was of the utmost importance to both sides. Lee desperately needed the supplies contained therein, in addition to the access the town provided to the Shenandoah Valley. McClellan needed to release the some 13,000 Union troops at the garrison while holding access to the river and rail junctions that were imperative supply routes. Again, a series of events led to a significant, though the under-appreciated battle of the Civil War. Lee marched to Maryland in order to expedite a Confederate victory; in order to do so he needed the supplies and arms contained in Harpers Ferry; in order to accomplish that task, it was necessary for him to divide his army. By a shear sheer stroke of happenstance, “McClellan was granted a windfall such as few generals in history have enjoyed,” and deployed his men in such a manner as the Battle of South Mountain had to ensue.<ref>McPherson, 108.</ref>[[File:gaps.jpg|thumbnail|300px|Troop movement in and around the Gaps.]]There were two major points by which to cross South Mountain: Crampton’s Gap and Turner’s Gap. Crampton’s was on the southwestern end of the mountain while Turner’s Gap was 12 miles to the north, with several small gaps in its near vicinity.<ref>Richard Slotkin, ''The Long Road to Antietam: How the Civil War Became a Revolution'' (New York: W.W. Norton& Company, 2012), 193.</ref>Orr’s gap was located three miles to the north of Turner’s while the Frosttown Gap was two miles south of Orr’s. Less than one mile south of Turner’s Gap was a mountain saddle known as Fox’s Gap. There were three primary roads (some were little more than foot pathsfootpaths) that ran through the gaps for crossing. The National Pike was the largest road in the region and crossed South Mountain at Turner’s Gap. The Burkittsville-Rohrersville Road ran over Crampton’s Gap while the Old Sharpsburg Road crossed the mountain at Fox’s Gap. Running across the peak of the mountain perpendicular to these roads was Wood Road in the northern region with Ridge Road picking up south of Fox’s Gap, where it continued southwest until intersection with the Loop road, which lived up to its name and turned back east to link with the Old Sharpsburg road.<ref>Hoptak, 36-41.</ref>Due to the a large number of routes around the mountain, only Turner’s, Fox’s, and Crampton’s Gaps actually crossed over the top.

The importance of the Battle of South Mountain cannot be overstated. This event was the first battle of the American Civil War fought on northern soil. Although at the end of the day Confederate troops still help the actual pass at Turner’s Gap, the battle was a resounding victory for the Union. If McClellan’s troops had actively chased Lee’s men into Sharpsburg rather than having passively followed them, the Battle of Antietam may have been avoided. By affording Lee’s troops the opportunity to convene in Sharpsburg and receive the message from Jackson that the siege at Harpers Ferry had gone in the favor of the Confederacy, McClellan provided Lee with a second chance; something no commander should ever have done.