Franklin's VI Corps attacked a small, hastily assembled Confederate force at Crampton's Gap in South Mountain that sought to protect the rear of Confederate Maj. Gen. Lafayette McLaws, who was across Pleasant Valley on Maryland Heights taking part in the siege of Harpers Ferry.

As part of that siege, units under Maj. Gen. Lafayette McLaws were sent to take Maryland Heights and then bombard the Union garrison in the town.

McClellan ordered Maj. Gen. William Franklin and his VI Corps to set out for Burkittsville from his camp at Buckeystown the following morning at daybreak, with instructions to drive through Crampton's Gap and attack McLaws' rear.

Once the VI Corps reached the summit they drove the Confederates from their positions, inflicting heavy casualties, in just fifteen minutes of fighting.

[2] After the Confederate line at the summit broke, the troops scattered in all directions into Pleasant Valley and, afterward, were completely unable to fight more against the Federals.

The following day Harpers Ferry surrendered to the Confederates, while Franklin sat camped in Pleasant Valley, convinced he was outnumbered by McLaws by nearly two to one.

Tactically, the Federals were successful in driving the Confederates from the gap while inflicting heavy casualties, the first time any portion of Lee's army had been driven from the field up until this point in the war.

The delay was long enough to ensure the safety of McLaws on Maryland Heights and capture of Harpers Ferry the following morning.

[citation needed] More significantly, after seizing Crampton's Gap, General Franklin failed to attack McLaws and allowed Stonewall Jackson's Corps to reunite with the main body of the Confederate army at Sharpsburg without a fight, setting the stage for the Battle of Antietam three days later.