The Battle of South Mountain (known in several early Southern accounts as the Battle of Boonsboro Gap) was fought September 14, 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 Army of Northern Virginia. Despite being significantly outnumbered, Lee"s army delayed McClellan"s advance for a day before withdrawing.

South Mountain is the name given to the continuation of the Blue Ridge Mountains after they enter Maryland. It is a natural obstacle that separates the Shenandoah Valley and Cumberland Valley from the eastern part of Maryland.

After Lee invaded Maryland, a copy of an order, known as order 191, detailing troop movements that he wrote fell into the hands of McClellan. From this, McClellan learned that Lee had split his forces and the Union general hoped to attack and defeat some of these isolated forces before they could concentrate against him. To reach Lee, McClellan had to move across South Mountain. Lee learned of McClellan"s intelligence coup and quickly sent forces to reinforce the passes to block his advance.

McClellan temporarily organized his army into three wings for the attacks on the passes. Maj. Gen. Ambrose Burnside, the Right Wing, commanded the I Corps (Maj. Gen. Joseph Hooker) and IX Corps (Maj. Gen. Jesse L. Reno). The Right Wing was sent to Turner"s Gap and Fox"s Gap in the north. The Left Wing, commanded by Maj. Gen. William B. Franklin, consisting of his own VI Corps and Maj. Gen. Darius N. Couch"s division of the IV Corps, was sent to Crampton"s Gap in the south. The Center Wing (II Corps and XII Corps), under Maj. Gen. Edwin V. Sumner, was in reserve.