The battle of Antietam, on September 17, 1862 at Sharpsburg, MD, resulted from the Army of the Potomac finding a copy of Robert E. Lee's orders during Lee's intended invasion of the north. In spite of catching Lee unprepared and divided, the Union army only managed a draw. Even so, this battle was the pivotal point in the war, ending the South's best chance of independence.
For a history of the Antietam campaign click here.
Click on the picture on the map, or on the link below the map, to see photographs from that portion of the battlefield.