After a copy of his plans detailing the widely scattered nature of his army fell into Union hands, Confederate general Robert E. Lee hurriedly concentrated his troops at Sharpsburg. His Union rival, George B. McClellan, pursued cautiously, assembling his army along Antietam Creek. There on September 17, the war’s bloodiest single-day battle was fought.

After a copy of his plans detailing the widely scattered nature of his army fell into Union hands, Confederate general Robert E. Lee hurriedly concentrated his troops at Sharpsburg. His Union rival, George B. McClellan, pursued cautiously, assembling his army along Antietam Creek. There on September 17, the war’s bloodiest single-day battle was fought.

Enjoying a nearly two-to-one advantage, McClellan planned to attack both flanks simultaneously; however, failure by the Union left to advance in the morning prevented a coordinated assault and allowed Lee to shift his outnumbered forces frequently throughout the day. No battle during the American Civil War exceeded Antietam’s intensity and ferocity. The confused fury of charges and countercharges on the Confederate left raged from dawn until almost noon. Action shifted to the center where combatants hammered away at each other along the “Bloody Lane” to the point of exhaustion. When the Union left finally drove toward Lee’s rear late in the afternoon, only the arrival and attack by the last Southern reinforcements saved Lee from disaster.

President Abraham Lincoln and General George B. McClellan visiting the Antietam battlefield two weeks after the battle. (National Archives)

There was no fighting the following day, but that evening, Lee withdrew across the Potomac. Union casualties exceeded 12,400 and Confederate losses topped 13,700.

Although a tactical draw, Antietam profoundly affected the war. Five days after the battle, President Abraham Lincoln issued his preliminary Emancipation Proclamation, broadening the war to include a moral crusade to free the slaves. In doing so, he effectively ended the prospect of foreign intervention.