American Civil War: Battle of South Mountain

Major General George B. McClellan. Photograph Courtesy of the National Archives & Records Administration

The Battle of South Mountain was fought September 14, 1862, and was part of the American Civil War's Maryland Campaign. Having moved north into Maryland following his victory at the Second Battle of Manassas, Confederate General Robert E. Lee hoped to conduct a prolonged campaign on Northern soil. This goal was spoiled when a copy of his marching orders, Special Order 191, fell into Union hands. Responding with unusual speed, Union commander Major General George B. McClellan put his army in motion to engage the enemy.

To block McClellan, Lee ordered troops to defend the passes over South Mountain in western Maryland. On September 14, Union troops attacked Crampton's, Turner's, and Fox's Gaps. While Confederates at Crampton's Gap were easily overwhelmed, those to the north at Turner's and Fox's Gaps offered stiffer resistance. Mounting assaults through the day, McClellan's men were finally able to drive off the defenders. The defeat forced Lee to curtail his campaign and re-concentrate his army near Sharpsburg. Moving through the gaps, Union troops opened the Battle of Antietam three days later.


In September 1862, Confederate General Robert E. Lee began moving his Army of Northern Virginia north into Maryland with the goal of severing the rail lines to Washington and securing supplies for his men. Dividing his army, he sent Major General Thomas "Stonewall" Jackson to capture Harper's Ferry, while Major General James Longstreet occupied Hagerstown. Pursuing Lee north, Union Major General George B. McClellan was alerted on September 13, that a copy of Lee's plans had been found by soldiers from the 27th Indiana Infantry.

Portrait of Robert E. Lee
General Robert E. Lee. Photograph Courtesy of the National Archives & Records Administration

Known as Special Order 191, the document was found in an envelope with three cigars wrapped in a piece of paper near a campsite recently used by Major General Daniel H. Hill's Confederate division. Reading the orders, McClellan learned Lee's marching routes and that the Confederates were spread out. Moving with uncharacteristic speed, McClellan began putting his troops in motion with the goal of defeating the Confederates before they could unite. To expedite passing over South Mountain, the Union commander divided his force into three wings.

Battle of South Mountain

  • Conflict: Civil War (1861-1865)
  • Date: September 14, 1862
  • Armies and Commanders:
  • Union
  • Major General George B. McClellan
  • 28,000 men
  • Confederates
  • General Robert E. Lee
  • 18,000 men
  • Casualties:
  • Union: 443 killed, 1,807 wounded, 75 captured/missing
  • Confederate: 325 killed, 1,560 wounded, 800 captured/missing

Crampton's Gap

The Left Wing, led by Major General William B. Frankin was assigned to capture Crampton's Gap. Moving through Burkittsville, MD, Franklin began deploying his corps near the base of South Mountain early on September 14. At the eastern base of the gap, Colonel William A. Parham commanded the Confederate defense which consisted of 500 men behind a low stone wall. After three hours of preparations, Franklin advanced and easily overwhelmed the defenders. In the fighting, 400 Confederates were captured, most of who were part of a reinforcement column sent to aid Parham.

Turner's & Fox's Gaps

To the north, the defense of Turner's and Fox's Gaps was tasked to the 5,000 men of Major General Daniel H. Hill's division. Spread over a two mile front, they faced the Right Wing of the Army of the Potomac led by Major General Ambrose Burnside. Around 9:00 AM, Burnside ordered Major General Jesse Reno's IX Corps to attack Fox's Gap. Led by the Kanawha Division, this assault secured much of land south of the gap. Pressing the attack, Reno's men were able to drive Confederate troops from a stone wall along the crest of the ridge.

Portrait of Ambrose Burnside
Major General Ambrose Burnside. Photograph Courtesy of the National Archives & Records Administration

Exhausted from their efforts, they failed to follow up this success and the Confederates formed a new defense near the Daniel Wise farm. This position was reinforced when Brigadier General John Bell Hood's Texas Brigade arrived. Re-commencing the attack, Reno was unable to take the farm and was mortally wounded in the fighting. To the north at Turner's Gap, Burnside sent Brigadier General John Gibbon's Iron Brigade up the National Road to attack Colonel Alfred H. Colquitt's Confederate brigade. Overrunning the Confederates, Gibbon's men drove them back up into the gap.

Widening the assault, Burnside had Major General Joseph Hooker commit the bulk of I Corps to the attack. Pressing forward, they were able to drive the Confederates back, but were prevented from taking the gap by the arrival of enemy reinforcements, failing daylight, and rough terrain. As night fell, Lee assessed his situation. With Crampton's Gap lost and his defensive line stretched to the breaking point, he elected to withdraw west in an effort to re-concentrate his army.

Portrait of Joseph Hooker
Major General Joseph Hooker. Photograph Courtesy of the National Archives & Records Administration


In the fighting at South Mountain, McClellan suffered 443 killed, 1,807 wounded, and 75 missing. Fighting on the defensive, Confederate losses were lighter and numbered 325 killed, 1,560 wounded, and 800 missing. Having taken the gaps, McClellan was in prime position to achieve his goal of attacking the elements of Lee's army before they could unite.

Unfortunately, McClellan reverted to the slow, cautious behavior which had been the hallmark of his failed Peninsula Campaign. Lingering on September 15, he provided time for Lee to re-concentrate the bulk of his army behind Antietam Creek. Finally moving forward, McClellan engaged Lee two days later at the Battle of Antietam.

Despite McClellan's failure to capitalize on the capture of the gaps, the victory at South Mountain provided a much needed victory for the Army of the Potomac and helped to improve morale after a summer of failures. Also, the engagement ended Lee's hopes for staging a prolonged campaign on Northern soil and put him on the defensive. Forced into making a bloody stand at Antietam, Lee and the Army of Northern Virginia were compelled to retreat back to Virginia after the battle.

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Your Citation
Hickman, Kennedy. "American Civil War: Battle of South Mountain." ThoughtCo, Aug. 26, 2020, Hickman, Kennedy. (2020, August 26). American Civil War: Battle of South Mountain. Retrieved from Hickman, Kennedy. "American Civil War: Battle of South Mountain." ThoughtCo. (accessed May 30, 2023).