How Did the Battle of South Mountain Alter the Course of the American Civil War

Map of The Maryland Campaign.

While the Battle of Antietam was the most important conflict that took place in Maryland, it critical to avoid overlooking the battle that forced the Union and Confederate forces to meet at Antietam Creek on September 14, 1862 along the gaps of South Mountain. An extension of the Blue Ridge Range, South Mountain was a heavily wooded and rocky terrain that ran southwest from Pennsylvania down to the Potomac River near Harpers Ferry. To the east of the mountain was the town of Frederick, Maryland, less than 50 miles from Washington, D.C.[1]

The fighting that occurred on the long Sunday was fierce and constant. Artillery, musket, bayonet, and fists were all employed as weapons, which resulted in a tremendous number of casualties. The Union forces engaged that day totaled 28,000 and by nightfall 2,325 were listed as casualties. The Confederate Army utilized 18,000 troops and suffered a loss of 2,685 men, an astounding 800 of which were listed as missing.[2]These men, many of whom are lost to history, engaged in a battle that led directly to the bloodiest single day in U.S. military history, Antietam, which in turn led to a new war aim for President Abraham Lincoln. The Battle of South Mountain, therefore, was the catalyst for the events that forever altered the course of the Civil War.

Lee Goes North

Robert E. Lee, 1863.

When Lee ordered his men out of Virginia, he had a massive force. The Confederate Army left Centreville, Virginia with 45,700 men. Additionally, Lee called up three divisions from Richmond that totaled another 20,600 troops in addition to J.E.B. Stuart’s cavalry of 5,500 riders. In total, 71,800 Confederate soldiers departed Virginia with orders to proceed to Maryland. General Thomas “Stonewall” Jackson was ordered to Harpers Ferry with his 20,000 men and 18,000 were engaged at South Mountain thereby leaving more than 33,000 troops for which there is no account. In a note from General Lee to Confederate President Jefferson Davis, he complained that his forces was “very much diminished̶̶̶ ̶ ̶ I fear from a third to one-half the original numbers.”[3]General Daniel Harvey (D.H.) Hill, who commanded a division of 8,000 troops who began their march in Richmond understood that “the want of shoes, the want of food, and physical exhaustion,” was a main factor in the rate of straggling, yet he had no empathy for the “thousands of thieving poltroons [who] had kept away from sheer cowardice.” He held the firm conviction that “the straggler is generally a thief and always a coward.”[4]Because of these “cowards”, who were in all likelihood seeking out food and rest, the Rebel forces were greatly diminished when they concentrated in Frederick on the ninth day of September.

On that night, Lee presented his plan of action to his generals, which he had written and issued as Special Orders No. 191. This directive, much to the consternation of Major General James Longstreet, split Lee’s army into 4 groups. Jackson was ordered to Harpers Ferry to seize the arsenal that was being held by a federal garrison under Colonel Dixon Miles. Longstreet was instructed to lead his troops north to Boonsboro and then Hagerstown, as there was faulty Confederate intelligence that there was movement among Federal troops in neighboring Pennsylvania. This left only Stuart’s cavalry and D.H. Hill’s men to guard the passes of South Mountain.[5]This was a well-crafted, albeit risky strategy, which became even more perilous once Union General George McClellan came into possession of Special Orders No. 191.

McClellan's Strategy

George B. McClellan

On September 13, three days after Lee’s troops moved out, McClellan’s army reached Frederick. While unaware that Lee had divided his army, McClellan believed Longstreet’s division to be in Boonsboro, wherein they were actually as far north as Hagerstown. McClellan’s initial strategy was to follow Longstreet northwest up the National Pike Road. These plans were altered, however, when by a great stroke of luck, McClellan came to possess Special Orders No. 191.

When the Army of the Potomac marched into Maryland, Corporal Barton W. Mitchell of the 27th Indiana came to rest on a field outside of Frederick. He found what at first appeared to be just three cigars on the ground. Upon further inspection he realized the cigars were wrapped in a paper with the words “Headquarters, Army of Northern Virginia, Special Orders, No. 191,” at the top.[6] Through the chain of command, the orders reached McClellan just before noon on September 13. The general wired President Lincoln with the message: “I think Lee has made a gross mistake, and that he will be severely punished for it…I Have the plans of the rebels, and will catch them in their own trap.”[7]He certainly had that ability, yet for reasons of unfounded over-cautiousness, he waited a full 18 hours before moving his men.

His plan was sound and if executed properly had the ability to cut Lee’s army in half and keep them separated. He planned on sending General Jesse Reno’s 9th Corps, followed by General Joseph Hooker’s 1st Corps to the northern mountain gaps known as Turner’s and Fox’s. General Edwin Sumner was to follow the 9th and 1st Corps with his Center Wing. Concurrently, he ordered William Franklin’s 6th Corps to cross over Catoctin Mountain, proceed through the village of Burkittsville, and then drive through South Mountain at Crampton’s Gap. From there he was to continue south to trap Confederate General Lafayette McLaws from the rear atop Maryland Heights, overlooking Harpers Ferry. McClellan believed that if he were able to affect the release of Colonel Miles’ troops at Harpers Ferry, they in turn could either reinforce the 9th and 1st Corps or trap Longstreet and D.H. Hill at the Potomac River.[8]The strategy was excellen,t yet the delay in action gave Lee time to reposition his troops on South Mountain in order to extend the protection of Jackson in Harpers Ferry.

Harpers Ferry and the Gaps

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 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 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.[9]

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.[10]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 footpaths) 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.[11]Due to a large number of routes around the mountain, only Turner’s, Fox’s, and Crampton’s Gaps actually crossed over the top.

The battles in and around these gaps raged on until darkness fell and fighting became impossible. By the end of the day, the Confederacy was able to maintain their position at Turner’s Gap, while the Union forces claimed possession of both Fox’s and Crampton’s. Although the Confederate Army was greatly outnumbered and McClellan was actually in possession of Lee’s plan prior to the battle, the Rebel soldiers were able to withstand the Federal assault until the end of the day, which afforded Jackson the time he needed to secure Harpers Ferry.

Lee's Retreat

The Battle of Antietam.

At ten o’clock on the night of September 14, the first Confederate troops made their way down the slope of South Mountain. Lee’s plans for a triumphant northern invasion withered as the sun rose. Just at that hour, a courier General Jackson had dispatched the previous evening reached Lee in Sharpsburg with the news that Harpers Ferry was on the brink of surrender and that Jackson’s troops would be free to support Lee and Longstreet. Upon hearing this news, Lee altered his plans to retreat and informed his generals, “We will make our stand on these hills.”[12]

The hills of which he spoke overlooked Antietam Creek in Sharpsburg, Maryland, which was the location of the single bloodiest day in U.S. military history. If several events had been altered at South Mountain, it is likely that the Battle of Antietam may never have taken place. If, for example, McClellan had acted with greater haste after intercepting Lee’s Special Orders No. 191, the Federals might have reached the South Mountain gaps prior to the Confederates gaining knowledge that McClellan was organizing his men for an assault.

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.


Aside from the mistakes made by the North, the Southern troops provided General Jackson the precious gift of time. Miraculously and against overwhelmingly stronger forces, the soldiers of the Confederacy occupied the Union troops long enough to distract them and detain them from converging on Harpers Ferry, thus affording Jackson the time he needed to take the garrison and provide Lee with the reinforcements he so desperately needed. By courageously holding their battle lines, the Confederate troops offered Lee the chance to halt his return to Virginia as a defeated army. Instead, through the fortitude of the Rebel soldiers, Lee had the chance to redeem himself in the eyes of the C.S.A. Perhaps more importantly, by affording Jackson time to complete his siege victoriously, the Confederate soldiers were the beneficiaries of the ample supplies stored at Harpers Ferry. In sum, without the Battle of South Mountain unfolding as it had, the course of the Civil War may have been dramatically different.


  1. John David Hoptak, The Battle of South Mountain (Charleston, S.C.: The History Press, 2011), 36.
  2. David J. Eicher, The Longest Night: A Military History of the Civil War (New York: Simon & Schuster, 2001), 344.
  3. United States War Department, War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies, 128 vols. (Washington, D.C.: Government Printing Office, 1888), series I, vol. 51, part I, 606. Hereinafter cited as OR.
  4. OR, series I, vol. 19, part I, 1026.
  5. Eicher, 339.
  6. James McPherson, Antietam: The Battle That Changed the Course of the Civil War (New York: Oxford University Press, 2002), 107.
  7. From OR, series I, vol. 19, part II, 281, quoted in McPherson, 109.
  8. Hoptak, 35.
  9. McPherson, 108.
  10. Richard Slotkin, The Long Road to Antietam: How the Civil War Became a Revolution (New York: W.W. Norton& Company, 2012), 193.
  11. Hoptak, 36-41.
  12. Joseph L. Harsh, Taken at the Flood: Robert E. Lee and Confederate Strategy in the Maryland Campaign of 1862 (Kent, OH: Kent State University Press, 1999), 305.

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