Anaxagoras
Banned
Special Order 191 was to be sent to Robert E. Lee for the plans of the invasion of the Potomac, the courier who was being sent to Lee for the order managed to be successful, and with the threat of a Union Ambush in the road he was travelling. The Battle of Antietam came under Confederate victory, and smashed the Army of the Potomac, by the autumn of 1862, the Confederates captured Washington D.C.
If Special Order 191 was not lost, then butterflies would have immediately resulted, and there would have been no Battle of Antietam at all.
In any case, I can see any scenario without ASB intervention that would allow the AoNV to win such a massive victory over the AotP that the fall of Washington would result.