Confederate General Robert E. Lee wrote Special Order No. 191 on September 9, 1862 during his Maryland Campaign. In it he outlined his plans for his Army of Northern Virginia during the campaign. In it he specifies dividing his army into pieces. Major General Stonewall Jackson would go to Harpers Ferry and capture supplies, Major General James Longstreet would head north to Boonsboro, and the main portion of Lee's army would march to Hagerstown.

Copies were made of the order for each of Lee's commanders, and additional copies were made for subordinate commanders. One of these copies, made for Major General Daniel Hill, was either discarded or accidentally dropped at Best Farm.

On September 13th, Union Private Barton Mitchell discovered the orders in an envelope wrapped around a bundle of cigars. He passed his find to his superiors and eventually the copy of the orders were given to Major General George McClellan, the commander of the Army of the Potomac.

General McClellan was delighted to learn of the Confederates planned troop movements. He is reported as saying, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home."

General Lee's and General McClellan's armies met on the battlefield at Antietam Creek near Sharpsburg, Maryland. The battle would be the bloodiest day of the Civil War. McClellan halted Lee's invation into the North, but did not pursue the Confederates, a decision which has been debated by historians.