Three pitched battles were fought for possession of three South Mountain passes: Crampton's, Turner's, and Fox's Gaps.

[1] Although the delay bought at South Mountain would allow him to reunite his army and forestall defeat in detail, Lee considered termination of the Maryland Campaign at nightfall.

[3] From this, McClellan learned that Lee had split his forces, sending one wing under Maj. Gen. Thomas J. Jackson to lay siege to Harper's Ferry.

[4] Lee hoped that after taking Harper's Ferry to secure his rear, he could carry out an invasion of the Union, wrecking the Monocacy aqueduct, before turning his attention to Baltimore, Philadelphia, or Washington, D.C..[5] To counter the Confederate invasion, McClellan led the Army of the Potomac west in an effort to force battle on the isolated parts of Lee's divided force.

From Boonsboro, Lee had sent a column under Maj Gen. James Longstreet northward to respond to a perceived threat from Pennsylvania.

[7] At the southernmost point of the battle, near Burkittsville, Confederate cavalry and a small portion of Maj. Gen. Lafayette McLaws's division defended Brownsville Pass and Crampton's Gap.

McLaws was unaware of the approach of 12,000 Federals and had only 500 men under Col. William A. Parham thinly deployed behind a three quarter-mile-long stone wall at the eastern base of Crampton's Gap.

They are on the north side of Alternate U.S. 40, the old National Pike, the main highway west from the Chesapeake Bay to the Ohio Valley during the Civil War.

[11] Just to the south, other elements of Hill's division (most notably Drayton's Brigade [12]) defended Fox's Gap against Reno's IX Corps.

Reno sent forward the rest of his corps, but due to the timely arrival of Southern reinforcements under Confederate Brig.

After Farmer Wise was paid one dollar each to bury the Confederate soldiers who died behind the stone walls on or near his property, sixty (or more) bodies were dumped down his dry well.

[24] In January 2011, the South Mountain battlefield gained National Heritage Landmark status and in August 2012, the State of Maryland declared its continued commitment to the preservation of these invaluable lands.

[25] The Civil War Trust followed up with another successful preservation victory in 2013 by saving 298 acres of battlefield land at Turner's Gap.

[26] However, nearby heritage areas in Frederick and Washington counties are still threatened by development and South Mountain was listed as one of the Most Endangered Battlefields in the 2009 and 2010 editions of History Under Siege.