General Robert E. Lee's exhausted Confederate forces hold off the pursuing Yankees by closing two passes through Maryland's South Mountain, allowing Lee time to gather his forces further west along Antietam Creek near Sharpsburg.
After the Second Battle of Bull Run, Virginia, in late August 1982, Lee decided to invade Maryland to raise supplies; he also hoped a decisive win would earn the South foreign recognition. As he moved, he split his army into five sections while the hungry Rebels searched for supplies. A copy of the Confederate plans accidentally fell into Union hands when the orders were left in an abandoned campsite outside of Frederick, Maryland. McClellan now knew that Lee's force was in pieces, but he was slow to react.
As Lee moved into western Maryland, he left detachments to guard Crampton's Gap and Turner's Gap through South Mountain. If McClellan had penetrated the passes, he would have found Lee's army scattered and vulnerable. South Mountain, a 50-mile-long ridge, contained several passes, but Crampton's Gap and Turner's Gap were the most important. The National Road ran through Turner's Gap to the north, and Crampton's Gap connected western Maryland to Harpers Ferry, Virginia.
The Union troops drove the Confederates away at Crampton's Gap, but were initially unable to expel the Confederates from Turner's Gap. However, the Rebels did retreat the next morning. Union losses for the day amounted to 2,300 dead and wounded, including the death of Major General Jesse Reno. The Confederates lost 2,700.
These engagements were a mere prelude to the Battle of Antietam. Although costly, they allowed Lee time to assemble his scattered bands at Sharpsburg.