14 September – On this day

The Battle of South Mountain – known in several early Southern accounts as the Battle of Boonsboro Gap – was fought on this day in 1862, as part of the Maryland Campaign of the American Civil War. Three pitched battles were fought for possession of three South Mountain passes: Crampton’s, Turner’s, and Fox’s Gaps.

Maj. Gen. George B. McClellan, commanding the Union Army of the Potomac, needed to pass through these gaps in his pursuit of Confederate General Robert E. Lee’s precariously divided Army of Northern Virginia. By dusk, with Crampton’s Gap lost and his position at Fox’s and Turner’s Gaps precarious, Lee ordered his outnumbered forces to withdraw from South Mountain. McClellan was now in position to destroy Lee’s army before it could concentrate. Union casualties of 28,000 engaged were 2,325; Confederates lost 2,685 of 18,000.

The Battle of South Mountain was an important morale booster for the defeat-stricken Army of the Potomac. The New York World wrote that the battle “turn[ed] back the tide of rebel successes” and “the strength of the rebels is hopelessly broken.” Lee contemplated the end of his Maryland campaign. However, McClellan’s limited activity on 15 September after his victory at South Mountain condemned the garrison at Harpers Ferry to capture and gave Lee time to unite his scattered divisions at Sharpsburg for the Battle of Antietam on 17 September.