Battle of Antietam
Also called: Battle of Sharpsburg
McClellan’s Union army stopped Lee’s invasion of the North at Sharpsburg, Maryland; with some 23,000 casualties it was the bloodiest single day in American history.

Alexander Gardner · Public domain · Source
In September 1862 Robert E. Lee carried the war into Maryland for the first time. Having found a copy of Lee’s orders, George McClellan attacked him along Antietam Creek on 17 September. Fighting raged all day in the Cornfield, the sunken road called “Bloody Lane” and at Burnside’s Bridge, and casualties on both sides reached about 23,000.
The battle was inconclusive, but Lee withdrew to Virginia and McClellan did not pursue. Lincoln seized the moment to issue the preliminary Emancipation Proclamation on 22 September. In October Alexander Gardner’s photographs of the dead went on show at Mathew Brady’s New York gallery, and for the first time the public saw the bodies of a battlefield in photographs.