September 14, 1862 – A portion of General Robert E. Lee’s Confederate Army of Northern Virginia defended key mountain passes in Maryland against Major General George B. McClellan’s Federal Army of the Potomac.
Sixteen hours after receiving Lee’s lost Special Order No. 191, McClellan finally moved to attack Lee’s scattered army. Part of Lee’s forces besieged the Federal garrison at Harpers Ferry, while another part guarded the path to Harpers Ferry and the army’s supply train through the passes of South Mountain, a part of the Blue Ridge chain about 20 miles south of the Pennsylvania border. McClellan planned to “cut the enemy in two and beat him in detail.”
The Federals descended on three mountain passes: Turner’s Gap in the north, Fox’s Gap in the center, and Crampton’s Gap in the south. A small Confederate force led by Major General D.H. Hill guarded Turner’s and Fox’s, which were essential to preserving the army’s supplies. The Federal army’s right wing–two corps numbering some 30,000 men under Major General Ambrose E. Burnside–advanced on these two gaps.
Fighting began when Major General Jesse L. Reno’s Federal IX Corps attacked Confederates in the pastures in front of Fox’s Gap around 9 a.m. Confederates led by Brigadier General Samuel Garland counterattacked, and both Reno and Garland were killed in action. The Federals captured most of Garland’s brigade, but their assault on Fox’s Gap stalled in the face of stubborn resistance and the arrival of Confederate reinforcements.
Meanwhile, the Federal I Corps under Major General Joseph Hooker advanced on the National road in Boonsboro to seize Turner’s Gap, a mile north of Fox’s. Confederate defenders resisted into the afternoon, but superior Federal numbers eventually overwhelmed them. As the Confederates began breaking, two divisions under Major General James Longstreet reinforced them and prevented a rout. Fighting continued until the Federals secured the high ground at Turner’s around 10 p.m., and the Confederates withdrew near midnight.
At Crampton’s Gap, the southernmost defile, the Federal left wing–Major General William B. Franklin’s 12,000-man VI Corps–advanced with orders to break through the pass and “cut off, destroy or capture (General Lafayette) McLaws’ command & relieve Col (Dixon) Miles” at Harpers Ferry, just a few miles away.
A single Confederate brigade under Colonel William A. Parham guarded the gap when the Federals attacked around noon. The Confederates held against repeated attacks for several hours, but they eventually wavered and broke. Brigadier General Howell Cobb arrived with Confederate reinforcements, but they soon wavered as well and joined the panicked retreat down the mountainside into Pleasant Valley.
The arrival of McLaws and his men from Harpers Ferry finally stabilized the Confederate line. Franklin, unaware that his troops could have easily destroyed the enemy, called a halt and camped for the night, missing a prime opportunity to relieve the garrison and cripple Lee’s army.
The battle ended in Federal victory, but the Confederates had held the mountain passes for most of the day against a force five times their size. This gave Lee the time he needed to concentrate his army. The Federals sustained 2,325 casualties (443 killed, 1,807 wounded, and 75 missing) out of about 28,000 effectives. The Confederates lost 2,685 (325 killed, 1,560 wounded, and 800 missing or captured) from some 18,000.
When Lee received news of the fight at South Mountain, he decided to return to Virginia. Word of Federals taking Crampton’s Gap and threatening McLaws’s rear at Harpers Ferry confirmed Lee’s decision. Lee sent a message to McLaws that night: “The day has gone against us and this army will go by Sharpsburg and cross the (Potomac) river.”
Lee sought to concentrate his army at Sharpsburg, the first stop on his withdrawal, about six miles west of South Mountain. But news from Major General Thomas J. “Stonewall” Jackson at Harpers Ferry soon compelled Lee to change his mind and make a stand.
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