In September 1862 two Union soldiers resting in a Maryland meadow found three cigars wrapped in a copy of Robert E. Lee’s Special Order 191, which showed his army divided to capture Harpers Ferry. Lee had invaded Maryland to feed his men and to win recognition from Britain and France. George McClellan, with about 87,000 men against Lee’s 38,000 but trusting Allan Pinkerton’s inflated estimates, waited 18 hours, giving Lee time to concentrate behind Antietam Creek at Sharpsburg.

The battle of September 17 unfolded in three separate phases. At dawn Joseph Hooker attacked through Miller’s cornfield toward the Dunker Church, and the corn changed hands about 15 times. At midday Union divisions assaulted D.H. Hill’s men in the Sunken Road, later called Bloody Lane, until Francis Barlow’s troops flanked it, but McClellan held back more than 20,000 reserves. In the afternoon Ambrose Burnside spent hours forcing a stone bridge over a fordable creek, then A.P. Hill arrived from Harpers Ferry at 3:30 p.m. after a 17-mile march and drove him back.

• The Louisiana Tigers lost about 323 of 500 men, and the 12th Massachusetts suffered 67 percent casualties.
• Joseph K.F. Mansfield massed his green troops in a dense column and was shot almost immediately.
• Father William Corby rode the line giving absolution to the Irish Brigade, which lost 540 men.
• The day produced over 22,000 casualties, and five days later Lincoln issued the preliminary Emancipation Proclamation.
• Alexander Gardner’s photographs of the dead drew silent crowds at a New York gallery a month later.