On September 13, 1862, the Army of the Potomac under George McClellan was in pursuit of Robert E. Lee's Army of Northern Virginia. Nine days before, Lee had begun his first invasion of the North. Near Frederick, Maryland, Corporal Barton Mitchell came upon three cigars wrapped in a piece of paper. Upon further investigation, Mitchell discovered that the piece of paper was a copy of Lee's orders to his generals given on September 9. McClellan was immediately informed of the miraculous find and exclaimed, "Now I know what to do!" Yet he did nothing.

When Robert E. Lee invaded Maryland, he expected the Union garrisons at Harper's Ferry and Martinsburg to be abandoned. Unfortunately for Lee, the garrisons remained. Lee did not feel like he could successfully continue his campaign if Union troops were blocking his communication with the Shenandoah Valley. On September 9, 1862, Lee ordered his army to split up in order to capture the Union garrisons. A daring move, Lee was counting on the known extreme cautiousness of McClellan. With these orders, Lee believed that Harper's Ferry would quickly be taken, and he could resume his push North. The fighting at Harper's Ferry would begin on September 12.

On September 13, McClellan had this crucial information which could have spelled the end for the Confederacy, yet true to form, he delayed. McClellan decided to conduct a two-pronged offensive against Lee; 16,000 to rescue the garrison at Harper's Ferry and 60,000 to exploit Lee's divided army. However despite his plan, McClellan did not secure the element of surprise. His troops were not ordered to move until the early morning of September 14, hours after he received the Lost Order. Lee's rearguard was able to hold off McClellan's advance at South Mountain on September 15; the same day that the Harper's Ferry garrison surrendered. By September 17, the majority of Lee's army, aside from A.P. Hill's division, which was still at Harper's Ferry, faced McClellan on the fields outside of Sharpsburg for the Battle of Antietam. By the end of the day, the final Union attack was thwarted by A.P. Hill and his men, who had marched all day to join the fight. On the night of September 18, Lee's men quietly moved back across the Potomac to fight another day. McClellan had said when he discovered the Lost Order, "Here is a paper with which if I cannot whip Bobbie Lee, I will be willing to go home." Although he would claim otherwise, McClellan did not whip Bobbie Lee.

Every book regarding the Maryland Campaign mentions Special Order 191 as if it were crucial to the outcome of the Battle of Antietam. However, McClellan's discovery of Lee's Lost Order had no lasting