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https://www.history.com/this-day-in-history/september-13/union-troops-discover-rebels-antietam-battle-plan

"Union soldiers find a copy of Confederate General Robert E. Lee’s orders detailing the Confederates’ plan for the Antietam campaign near Frederick, Maryland. But Union General George B. McClellan was slow to act, and the advantage the intelligence provided was lost.

On the morning of September 13, the 27th Indiana rested in a meadow outside of Frederick, Maryland, which had served as the site of a Confederate camp a few days before. Sergeant John Bloss and Corporal Barton W. Mitchell found a piece of paper wrapped around three cigars. The paper was addressed to Confederate General D.H. Hill. Its title read, “Special Order No. 191, Headquarters, Army of Northern Virginia.” Realizing that they had discovered a copy of the Confederate operation plan, Bloss and Mitchell quickly passed it up the chain of command. By chance, the division adjutant general, Samuel Pittman, recognized the handwriting on the orders as that of a colleague from the prewar army, Robert Chilton, who was the adjutant general to Robert E. Lee.

Pittman took the order to McClellan. The Union commander had spent the previous week mystified by Lee’s operations, but now the Confederate plan was clear. He reportedly gloated, “Here is a paper with which if I cannot whip Bobbie Lee, I will be willing to go home.” McClellan now knew that Lee’s forces were split into five parts and scattered over a 30-mile stretch, with the Potomac River in between. At least eight miles separated each piece of Lee’s army, and McClellan was just a dozen miles from the nearest Confederate unit at South Mountain. Bruce Catton, the noted Civil War historian, observed that no general in the war “was ever given so fair a chance to destroy the opposing army one piece at a time.”

Yet McClellan squandered the opportunity. His initial jubilation was overtaken by his caution. He believed that Lee possessed a far greater number of troops than the Confederates actually had, despite the fact that the Maryland invasion resulted in a high rate of desertion among the Southerners. McClellan was also excruciatingly slow to respond to the information in the so-called Lost Order. He took 18 hours to set his army in motion, marching toward Turner’s Gap and Crampton’s Gap in South Mountain, a 50-mile long ridge that was part of the Blue Ridge Mountains.'... (more)