Antietam was the Bloodiest One Day Battle in American History. 23,000 soldiers were killed, wounded or missing after twelve hours of savage combat on September 17, 1862. The Battle of Antietam ended the Confederate Army of Northern Virginia's first invasion into the North and led to Abraham Lincoln's issuance of the preliminary Emancipation Proclamation. The Confederate Army of Northern Virginia invaded Maryland had been in Frederick City the week before. The Battle of South Mountain was fought on September 14, 1862. The Union Army of the Potomac had knowledge of Gen. Lee's entire battle plan because of finding the famous "lost Order 121" in a bivouac area used by Lee's Army and (them) then used a few days later by the pursuing Union troops. The area where it was found is contained within the Monocacy Battlefield Park.
Gen. Lee had split his Army to capture the Federal garrison at Harpers Ferry, so as not to leave the enemy in his rear. With the Union forces advancing against him he had to fight a delaying action in the passes on South Mountain and go on the defensive at Antietam to allow time for the capture of Harpers Ferry and his Army to be reunited.