In A Nutshell
In September 1862, Confederate Army General Robert E. Lee drafted an order that detailed the movement of the Northern Virginia Army during its invasion of Maryland. The document has become known as Special Order 191. On September 13, 1862, Corporal Barton W. Mitchell of the 27th Indiana Volunteers happened upon a copy of the document near a campground. The Union Army then used the information to help predict the movements of the Confederates and gained a major advantage in the American Civil War.
The Whole Bushel
The Maryland Campaign of the American Civil War was waged from September 4–20, 1862. It was the first invasion of the North by Robert E. Lee and the Confederate troops. On or around September 9, Lee drafted a document, Special Order 191, that detailed the movements of the Army of Northern Virginia as they attempted to capture Maryland. The document gave specific instructions for the Confederate Army, as Lee planned to divide the troops into pieces before staging an attack. Special Order 191 included information on the routes, roads, and timing of the Confederate plan. The document was copied by Adjutant Robert H. Chilton and handed out to a select few Confederate generals.
On September 13, 1862, Union Corporal Barton W. Mitchell of the 27th Indiana Volunteers discovered a bundled up piece of paper with three cigars inside of it 3 kilometers (2 mi) south of Frederick, Maryland. The area was recently vacated by Major General D. H. Hill of the Confederate Army and the order was addressed to General Hill. After investigation, Mitchell realized that the document included information on the movement of Confederate troops and was a copy of Special Order 191. He turned the document over to his sergeant, who made sure that it was viewed by Major General George B. McClellan. McClellan was overcome with glee by the intelligence and put together a plan to attack the isolated forces of Robert E. Lee, which ultimately led to the Battle of South Mountain on September 14.
McClellan used the information to track the Confederates and, on September 17, 1862, the Union Army launched an attack which started the Battle of Antietam. During the fight, the Union was able to take defensive positions behind Antietam Creek. The battle became the bloodiest single-day conflict in American history with 22,717 people killed. It shifted the momentum of the war to the Union side, and Robert E. Lee was forced to retreat. The victory caused Abraham Lincoln to issue the Emancipation Proclamation. In an interesting twist, if McClellan had not been tipped off by Special Order 191, history could have been different.