Loath as I am to generally take over this thread with McClellan stuff, I do think there's one that I should point to because I think it is a persistent misconception or myth and it is one that is fairly easy to disprove.
That is, of course, the Special Order 191 stuff.
There are two components to this. The first is the idea of an 18 hour delay, which is a myth; the second is the idea that SO191 offered the possibility to destroy Lee's army, which is a misconception.
To the myth, first.
The exact timing of when SO191 was captured is not quite clear, but the common story (i.e. a component of the myth) is that it was found around 10AM and already in McClellan's hands by noon, and the main evidence for this is the telegram he sent to Lincoln mentioning that he had the Rebel plans.
This telegram, however, does not say "12 noon", or "12 midday" or "12 Meridian". It says "12M", and in one copy "12 Midnight":
With the time of receipt 2:35 AM, as seen in the top left corner.
The commander of the 27th Indiana said that his unit's march did not end until "about noon" and that he was given the Lost Order shortly thereafter; it then took a while to work its way up the chain of command until McClellan recieved it, and we know he
did have it by about 3PM because that is when he ordered his cavalry to verify the Lost Order from observations at the front. McClellan's cavalry had already taken the pass at Braddock Heights around 1PM and the other pass through the Catoctins, Jefferson Pass, was gained around 5:30 PM (the taking of the passes is referred to in the telegram McClellan sent to Lincoln).
The first unit ordered forwards was 9th Corps; Cox said his division was marching out of Frederick around noon, and the 9th Corps as a whole was ordered forward around 3:30 PM. Part of 6th Corps began moving around 8PM in a two-hour night march (9th Corps also engaged in night marching) but the rest of the army was under orders to move at sunrise, because there was only one road through Frederick MD and it was currently a massive traffic jam.
1st Corps woke up at 3AM (to follow 9th Corps once they'd cleared the road), the advance by the main army began at dawn (around 6AM) and the attack on the Rebel rear-guard at South Mountain began around 9AM.
The idea that McClellan delayed by 18 hours is almost impossible. In the space of the 23 hours from 10AM on the 13th of September to 9AM on the 14th of September McClellan's army went from just entering Frederick to launching an attack at South Mountain, a distance of about 13 miles; it's not really plausible that they moved that far in just five hours (two of them at night), even before we consider all the evidence that the leading parts of the army were on the move well before the termination of any 18 hour delay.
Now for the misconception.
This is what the Lost Orders said, plus what Lee had actually done, and the opportunities that it opened for McClellan.
Note by the way that SO 191 was issued on Tuesday 9 September; consequently "Friday" is September 12. McClellan recieves the orders September 13.
Special Orders, No. 191
Hdqrs. Army of Northern Virginia
September 9, 1862
- (missing)
- (missing)
- The army will resume its march tomorrow, taking the Hagerstown road. General Jackson's command will form the advance, and, after passing Middletown, with such portion as he may select, take the route toward Sharpsburg, cross the Potomac at the most convenient point, and by Friday morning take possession of the Baltimore and Ohio Railroad, capture such of them as may be at Martinsburg, and intercept such as may attempt to escape from Harpers Ferry.
- General Longstreet's command will pursue the same road as far as Boonsborough, where it will halt, with reserve, supply, and baggage trains of the army.
- General McLaws, with his own division and that of General R. H. Anderson, will follow General Longstreet. On reaching Middletown will take the route to Harpers Ferry, and by Friday morning possess himself of the Maryland Heights and endeavor to capture the enemy at Harpers Ferry and vicinity.
- General Walker, with his division, after accomplishing the object in which he is now engaged, will cross the Potomac at Cheek's Ford, ascend its right bank to Lovettsville, take possession of Loudoun Heights, if practicable, by Friday morning, Key's Ford on his left, and the road between the end of the mountain and the Potomac on his right. He will, as far as practicable, cooperate with General McLaws and Jackson, and intercept retreat of the enemy.
- General D. H. Hill's division will form the rear guard of the army, pursuing the road taken by the main body. The reserve artillery, ordnance, and supply trains, &c., will precede General Hill.
- General Stuart will detach a squadron of cavalry to accompany the commands of Generals Longstreet, Jackson, and McLaws, and, with the main body of the cavalry, will cover the route of the army, bringing up all stragglers that may have been left behind.
- The commands of Generals Jackson, McLaws, and Walker, after accomplishing the objects for which they have been detached, will join the main body of the army at Boonsborough or Hagerstown.
- Each regiment on the march will habitually carry its axes in the regimental ordnance—wagons, for use of the men at their encampments, to procure wood &c.
(maps from the GCACW series, which show the roads quite well, with my own additions)
As of when the orders are captured, it's Saturday. The orders expired on Friday, but because Harpers Ferry has not yet fallen they are a fairly good predictor (though not perfect) of Confederate positions.
Given that McClellan knew DH Hill left Frederick on the 11th, and assuming that he knew there was no extra division or so knocking about not included in the orders, the below is what McClellan could reasonably predict Lee's positions to be in
red (along with the true positions, which are slightly different and where different are in
orange).
Note that while the cavalry is not marked on this map, this is because McClellan was actually in contact with it already - Stuart's cavalry was holding the Cactotin passes as of noon on the 13th, and Jefferson Pass is not taken until near sundown.
Before continuing, stop a moment and work out how you think there is an opportunity to destroy Lee's army.
As I see it:
Jackson's force and Walker's force (totalling 16 brigades all told) are impossible to catch as things stand. They have a free and clear escape route back south and to get at them means crossing the Potomac.
Longstreet's and DH Hill's forces (9 and 5 brigades respectively) are north of the Potomac and can be attacked, but they have a clear escape route to the west via Williamsport or Shepherdstown.
McLaws' force (10 brigades) is vulnerable
so long as Harpers Ferry holds out, as this pins them in the Pleasant Valley if South Mountain can be forced. This is about a quarter of Lee's force and is the component that is actually vulnerable.
With this in mind, what McClellan actually does is:
There's only two suitable roads to use, with two thirds of McClellan's army trying to squeeze through Frederick (thus a lot of the delay) and the attack at South Mountain begins around 9AM on the 14th.
To my mind this is about as good as it gets in terms of being able to exploit Special Order 191. Lee reacts quickly to try and make the situation better, and what's more Lee actually gets really lucky with Harpers Ferry surrendering when it does (he'd ordered Jackson to abandon the siege, but Jackson ignored orders) but the simple fact is that 3/4 of Lee's army is either safe from McClellan's attack or has a clear line of retreat to safety.
That's why I say it's a misconception that McClellan could have exploited SO191 to destroy Lee's army.