On September 9, 1862, as Robert E. Lee’s Army of Northern Virginia marched into Maryland, after its victory at Second Manassas, the Confederate commander issued Special Order 191. The document split Lee’s army into three separate attacking columns. Stonewall Jackson would take the bulk of the force to surround and capture the Union garrison at Harpers Ferry, securing Lee’s supply line through the Shenandoah Valley. Other columns would move toward Hagerstown and Boonsboro. Lee expected to reunite his scattered commands before the cautious George B. McClellan, commanding the Union Army of the Potomac, could close with him.

Four days later, on the morning of September 13, soldiers of the 27th Indiana Volunteer Infantry halted in a meadow outside Frederick. Corporal Barton Mitchell spotted three cigars wrapped in a sheet of paper. The paper was a copy of Special Order 191, apparently dropped by a Confederate courier. It quickly traveled up the chain of command and reached McClellan that afternoon. The Union commander, who until then had been uncertain of Lee’s intentions, now possessed the enemy’s operational plan. “Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home,” he declared. He telegraphed Washington that he had “all the plans of the rebels.”

The intelligence prompted McClellan to advance more rapidly than Lee anticipated. On September 14, Union forces attacked the mountain gaps at South Mountain. The fighting bought Lee precious hours, with Major General D.H. Hill’s Division plugging three mountain gaps. Jackson completed the capture of Harpers Ferry on the 15th, and most of the Army of Northern Virginia concentrated behind Antietam Creek, near Sharpsburg. What Lee had hoped would be a campaign of maneuver, on ground of his choosing, became a defensive stand.

On September 17, the two armies collided in the Battle of Antietam, the bloodiest single day in American history. More than 22,000 men fell. McClellan’s habitual caution, and incomplete commitment of his larger force, allowed Lee’s outnumbered army to hold its lines, through a day of savage fighting in the Cornfield, Bloody Lane, and at Burnside’s Bridge. The following day McClellan declined to renew the attack. Lee withdrew across the Potomac River on the night of the 18th, his first invasion of the North ended but his army, while severely decimated, was still intact.

Lee later called the loss of the order a “great calamity,” believing that two or three more days of McClellan’s usual deliberation would have allowed him to rest his men, collect stragglers, and fight on better terms. Historians still debate how fully McClellan exploited the windfall; some argue he was already moving and that the orders were several days old. Yet the discovery clearly accelerated the campaign’s climax and forced Lee into a battle he had not sought at that moment or place. The resulting “tactical draw” became a strategic Union success. It halted the Confederate offensive and gave President Abraham Lincoln the opportunity to issue the Preliminary Emancipation Proclamation, five days later, transforming the character of the war. A single lost order, found by chance on September 13, had helped reshape the Maryland Campaign and the conflict itself.

The finger pointing, from Washington City, would continue for weeks. President Lincoln was exasperated by his deliberate planning, post Antietam, and would travel to Sharpsburg, on October 1, to meet with his commander. He would met with McClellan on October 3, after reviewing the Army of the Potomac. Lincoln knew McClellan had a case of the “slows,” and ordered General-In-Chief Henry Halleck to order McClellan to pursue Lee – an order which McClellan received on October 6. McClellan did indeed have the “slows.” His army would remain in Sharpsburg until October 26.