Source: Library of Congress.
In mid-September 1862, after the loss at Second Manassas/Bull Run, the Army of the Potomac shadowed the Army of Northern Virginia as Robert E. Lee began his first major incursion into the North. While at Frederick, MD, George McClellan obtained a copy of the Confederate battle plan--Lee's Special Order No. 191--that had been found wrapping three cigars. Saying that if he could not whip "Bobby Lee" with these plans in hand, he should be removed, he sped-up his then in progress western movement. The Army of the Potomac fought battles on 14 September 1862, for the South Mountain passes at Turner's, Fox's, and Crampton's Gaps before meeting the Army of Northern Virginia just east of Sharpsburg, MD.
The Battle of Sharpsburg/Antietam (the Confederates tended to name battles after the nearest town or landmark while the Union often named them for the nearest body of water) that took place on 17 September 1862 was really three separate battles on the same battlefield. The first battle, begun at 6 AM, took place on the Confederate left flank and was centered on the woods and cornfield near the Dunker Church, just north and slightly to the east of Sharpsburg, MD. The second battle, beginning in the mid-morning and lasting until early-afternoon, took place to the east of Sharpsburg, in the center of the Confederate lines in the vicinity of a sunken road, later called "Bloody Lane." The final stage of the battle took place on the right of the Confederate lines at the Rohrbach or Lower Bridge,one of three bridges crossing Antietam Creek in the vicinity of Sharpsburg.
When it was all over, it was the bloodiest single day of the war. McClellan's Army of the Potomac sustained 12,410 casualties, double those of D-Day on 6 June 1944, and Lee's Army of Northern Virginia lost 10,700 men, one-fourth of the Army.
Although technically a very bloody draw, Lee's leaving McClellan on the field allowed the Union, by the standards of the day, to declare it a victory. It was all President Abraham Lincoln needed to announce his Preliminary Emancipation Proclamation on 22 September 1862. The lack of a clear victory for the Confederacy, along with the Emancipation Proclamation, also caused Great Britain to rethink recognizing the Confederate States of America.