Okay, here's a idea for you…
On September 13, 1862, some unlucky Confederate soldier lost a copy of Lee's Special Order 191, which was discovered by Union Corporal Barton W. Mitchell in a field outside Frederick, Maryland. S.O. 191 was bucked up the chain of command until it reached McClellan, who realized he could destroy General Lee's Army of Northern Virginia, as it was divided into five parts, and each was roughly 10 miles separated from one another, including units which were widely separated by the Potomac River and about 30 miles apart from each other.
McClellan gleefully stated that if he could not whip Bobbie Lee with the recovered orders, he would quit and go home. But McClellan, although he was a skilled organizer and much beloved by his troops, also had a habit of moving cautiously, and did not make his move until September 17, while during that time, Lee was able concentrate his forces of Sharpsburg, while Jackson was able to take Harpers Ferry on September 15. After the Battle of Antietam, Lee was forced to retreat on September 18.
Now, suppose McClellan decided to abandon his usual cautiousness for once and acted decisively, moving as soon as his forces were ready. Could he have succeeded in destroying the Army of N. Virginia, or at least knock out a significant portion of it? If he had, by how much would this have shortened the war - or could he have ended it right then and there? And how would the future play out afterwards?