On the Confederate side, General Robert E. Lee was hoping to strike a decisive blow by invading the North. Lee's plan was to strike into Pennsylvania, imperiling the city of Washington and forcing an end to the war.
The Confederate Army began crossing the Potomac on September 4, and within a few days had entered Frederick, a town in western Maryland. The citizens of the town stared at the Confederates as they passed through, hardly extending the warm welcome Lee had hoped to receive in Maryland.
Lee split up his forces, sending part of the Army of Northern Virginia to capture the town of Harper's Ferry and its federal arsenal (which had been the site of John Brown's raid three years earlier).
Gen. George McClellan Moved the Army of the Potomac to Confront Robert E. Lee
Union forces under the command of General George McClellan began moving northwest from the area of Washington, D.C., essentially chasing the Confederates. At one point the Union troops camped in a field where the Confederates had camped earlier, and in an astounding twist, a copy of Lee's orders detailing how his forces were divided was discovered by a Union sergeant and taken to the high command.
General McClellan thus had invaluable intelligence, the precise disposition of Lee's forces. But McClellan, whose fatal flaw was that he was always overly cautious, did not fully capitalize on the information in his possession.
McClellan continued in his pursuit of Lee, who began consolidating his forces and preparing for a major battle.
The Battle of South Mountain
On September 14, 1862, the Battle of South Mountain, a struggle for mountain passes which led into western Maryland, was fought. The Union forces finally dislodged the Confederates, who retreated back into a region of farmland between South Mountain and the Potomac River.
Lee arranged his forces in the vicinity of Sharpsburg, a small farming village near the Antietam Creek.
On September 16 both armies took up positions near Sharpsburg and prepared for battle.
On the Union side, General McClellan had more than 80,000 men under his command. On the Confederate side, General Lee's army had been diminished by straggling and desertion on the Maryland campaign, and numbered approximately 50,000 men.
As the troops settled into their camps on the night of September 16, 1862, it seemed clear that a major battle would be fought the next day.