I know there's a whole novel series about this, but I thought it was worth looking into in greater detail anyway. Historically Special Order 191 was found somewhat after noon on September 13th and handed in to McClellan a couple of hours later. The situation on the night of the 12th was: https://www.loc.gov/resource/g3841sm.gcw0242000/?sp=6&r=0.12,0.16,0.615,0.396,0 Since then McClellan had entered Frederick, and he'd launched Burnside's wing against South Mountain - indeed, he was handed SO191 after seeing Cox's division off (they were the vanguard). Historically, Burnside's wing deployed against South Mountain (Fox's Gap and Turner's Gap) beginning at dawn on the 14th, after a partial night march. So... ...the question is, what would have resulted if SO191 wasn't lost? McClellan's army is already marching to contact when things change, after all... As far as I can see, there are a few possibilities: 1) McClellan sends part of his army over the Cacotins while keeping the rest in Frederick, and that wing gets cut off and destroyed. This seems fundamentally unlikely to me - there's no reason for this to happen. 2) McClellan sends his forces only after Fox's and Turner's and doesn't force Crampton's Gap, thus Lee's managed to hold South Mountain against him. This results in a stalemate, but it's one which works both ways - McClellan can hold South Mountain with only a portion of his own army and send the rest south to try and cut Lee off. 3) McClellan forces South Mountain. This is back on the historical track, and Antietam goes more or less as it did historically. 3a) McClellan forces South Mountain but thinks the Confederates have reconcentrated. This might actually result in a better outcome for the Union than historical - this would lead to McClellan not attacking until the 18th (when his own forces were mostly up) and having a more favourable force ratio. Any options I've missed?