South Mountain

On September 14, 1862, Union and Confederate forces clashed at South Mountain in an engagement that some early Southern accounts instead called the Battle of Boonsboro Gap. This fight formed part of the larger Maryland campaign of the Civil War. Possession of three mountain passes, Crampton's, Turner's, and Fox's Gaps, was contested through separate pitched battles.

The Union commander, Maj. Gen. George B. McClellan, led the Army of the Potomac and required passage through these gaps to continue chasing Confederate General Robert E. Lee, whose Army of Northern Virginia was dangerously split at the time. The stand at South Mountain cost precious hours, and those hours ultimately gave Lee the chance to bring his scattered forces back together and avoid being beaten piecemeal. Even so, by nightfall Lee was weighing whether to end the Maryland Campaign altogether.

Background

When the Blue Ridge Mountains cross into Maryland, they continue on as South Mountain, forming a natural barrier that divides the Hagerstown Valley and Cumberland Valley from Maryland's eastern half.

Lee's move into Maryland set off a chain of events after a copy of his order outlining troop movements, Special Order 191, ended up in McClellan's possession. The document revealed that Lee had divided his army, dispatching Maj. Gen. Thomas J. Jackson and one wing to besiege Harper's Ferry while the remaining forces waited at Boonsboro under Maj. Gen. James Longstreet. Lee's plan called for securing his rear by capturing Harper's Ferry, then pushing into Union territory and destroying the Monocacy aqueduct before setting his sights on Baltimore, Philadelphia, or Washington, D.C.

Determined to respond to this threat, McClellan pushed the Army of the Potomac westward, hoping to catch Lee's scattered divisions before they could reunite.

For the assault on the mountain passes, McClellan divided his army into three temporary wings. The Right Wing, led by Maj. Gen. Ambrose Burnside, included the I Corps under Maj. Gen. Joseph Hooker and the IX Corps under Maj. Gen. Jesse L. Reno, and moved toward Turner's Gap and Fox's Gap to the north. To the south, Maj. Gen. William B. Franklin commanded the Left Wing, made up of his own VI Corps along with Maj. Gen. Darius N. Couch's division from the IV Corps, and this force headed for Crampton's Gap. Meanwhile, Maj. Gen. Edwin Vose Sumner held the Center Wing, comprising the II Corps and XII Corps, in reserve.

Longstreet's column, which Lee had originally sent north from Boonsboro to deal with what seemed like a threat coming from Pennsylvania, was hastily called back once Lee discovered that his plans had fallen into enemy hands. Lee needed Longstreet's men to defend the passes through South Mountain and slow McClellan's approach. As the battle began, however, Boonsboro itself held only a single Confederate division, five brigades strong, under the command of Maj. Gen. D.H. Hill.

Battles

Near Burkittsville, at the southern edge of the fighting, Confederate horsemen along with a limited detachment from Maj. Gen. Lafayette McLaws's division held Brownsville Pass and Crampton's Gap. McLaws did not realize that 12,000 Union troops were closing in, leaving Col. William A. Parham with a mere 500 men spread thin along a stone wall three-quarters of a mile long at the base of the gap's eastern side. Franklin took three hours arranging his troops for the assault, prompting one Confederate to remark afterward that it resembled a lion taking exceedingly careful preparations before pouncing on a plucky little mouse. He placed Maj. Gen. Henry Warner Slocum's division on his right flank and Maj. Gen. William F. "Baldy" Smith's on his left. The gap fell to these forces, and roughly 400 pr

Future presidents

Among the men engaged at Fox's Gap during the Battle of South Mountain were two soldiers who later occupied the nation's highest office: Rutherford B. Hayes and William McKinley, both serving with the 23rd Ohio Infantry. Hayes led the regiment, while McKinley served as a commissary sergeant. By war's end, Hayes had risen to brevet major general and McKinley to brevet major. Their political paths eventually converged on the presidency, though separated by two decades, with Hayes winning the office in 1876 and McKinley following in 1896.

Aftermath

As evening fell, Crampton's Gap had already slipped away and the situation at Fox's and Turner's Gaps grew too dangerous to hold, so Lee gave the command for his outnumbered troops to pull back from South Mountain. This left McClellan poised to crush Lee's army before its scattered pieces could come together. The toll on the Union side, out of 28,000 men committed, reached 2,325 casualties, made up of 443 dead, 1,807 injured, and 75 unaccounted for. Confederate losses stood at 2,685 out of a force of 18,000, comprising 325 killed, 1,560 wounded, and 800 missing. For an army worn down by repeated setbacks, this fight at South Mountain served as a genuine lift in spirits for the Army of the Potomac. According to the New York World, the clash had begun "turn[ing] back the tide of rebel successes," leaving "the strength of the rebels hopelessly broken." Lee, for his part, began weighing whether his Maryland campaign had reached its close. Yet McClellan failed to press his advantage on September 15 following the victory, and that hesitation sealed the fate of the Harpers Ferry garrison, which fell into Confederate hands. It also bought Lee the time he needed to bring his separated divisions together at Sharpsburg, setting the stage for the Battle of Antietam on September 17.