The Battle of South Mountain erupted on September 14, 1862, when elements of the Union army tried to drive the Confederate rear guard from Crampton’s, Fox’s, and Turner’s Gaps and break through to the western side of the mountain to attack Confederates there. Earlier that month, when Confederate Gen. Robert E. Lee and his Army of Northern Virginia marched into Maryland, he was seeking supplies and recruits for a possible invasion of Pennsylvania. While resting men at Frederick, he hoped the Union garrison at Harpers Ferry would flee and leave his lines of communication and transportation unhindered. When the garrison remained in place and threatened his rear, Lee issued Special Orders No. 191 to divide his army and send Stonewall Jackson with about half of it to capture Harpers Ferry. Lee then marched over South Mountain, using it as a screen to help conceal the rest of his troops and keep Gen. George B. McClellan’s pursuing Army of the Potomac at bay. After the Federals found a dropped copy of Special Orders No. 191, it became especially critical for the outnumbered Confederates to hold the South Mountain passes until Jackson completed his mission and rejoined them. The day-long battle gave the Confederates just enough time.
