Very Informative Article on How McClellan Outsmarted Lee at Antietam

In relation to Antietam, by the way, the equivalent for the Seven Days would be (roughly) that the first half of the Seven Days sequence is the events up to the 17th (i.e. the attacking general manoeuvres against the defending general and creates a situation compelling him to withdraw).

If the rest of Antietam had gone like the Seven Days, Lee would have manoeuvred to get back across the Potomac in a position from which he could subsequently threaten Washington or Baltimore (that's the Richmond or Petersburg parallel); if the rest of the Seven Days had gone like Antietam, McClellan would have been forced back across a river line which Lee would then hold in force.

It does seem a bit odd that after both Antietam and the Seven Days there was about a month's pause, but that McClellan is the one who gets the blame in both cases!
 
Eh? McClellan attack at Oak Grove was a shambles , After that Lee took the initiative with some very poorly coordinated attacks but still managed to convince McClellan he was completely outnumbered but all I see are people Croaking on about how many casualties McClellan inflicted while basically giving up all the ground the A0P had gained yep what a genius.

I'm also tired of people defending McClellan and not giving Lee the credit for whipping his behind in the The Seven Days Battle's ohh wait they did most sane Historians all agree McClellan bottled it.

Oak Grove has an attack which gained about a mile of ground. The next day McClellan attacked north at gained Garnett's Hill. This placed the national army within striking range of Richmond.

Lee crossed the Chickahominy, leaving 40,000 men in the Richmond defences (i.e. more than faced Grant in '64) and attacked at Beaver Dam Creek, and was slaughtered. The position was forced to be abandoned when Jackson arrived from the Valley and threatened the rear of the Federal line. Lee by then had the numerically superior force.

Once McClellan's line is turned he has no choice but to pull back to a new one. This is conducted skillfully to Boatswain's swamp, and the forces there reinforced. However, it is impossible to cover his supply line. Men have to eat, and so McClellan skillfully moves his army to a position where they can eat.
 
When?



What timidity? The Seven Days Battle began with McClellan attacking. When Lee counterattacked, with superior numbers, McClellan carried out a brilliant tactical retreat and moved to a location he had been wanting to occupy for weeks anyway-- Harrison's Landing--and in the process inflicted horrific casualties on Lee's army.

Once secured at Harrison's Landing, McClellan was anxious to resume his advance. Quite naturally and justifiably, after fighting six battles in seven days, he asked Washington for reinforcements and made clear that he was willing and ready to resume his advance on Richmond.

Furthermore, at Harrison's Landing, McClellan was in a much better position to advance on Richmond than he had been when straddling the Chickahominy River, even though he was farther away. Even Commodore Goldsborough argued that McClellan should be allowed to resume his advance.

When McClellan realized that Washington simply was not going to give him anything close to the reinforcements he had requested, he deployed a reconnaissance in force in preparation for resuming his advance and drove the Confederates from Malvern Hill. But, in spite of this positive action, Lincoln ordered him to hand over his army to Pope, which freed Lee to send a sizable force against Pope and led to the debacle at Second Bull Run.
I was not aware of McCellan deploying a reconnaissance force and driving the conferates from Malvern Hill was this launched from Harrison's Landing?
 
I was not aware of McCellan deploying a reconnaissance force and driving the conferates from Malvern Hill was this launched from Harrison's Landing?
Yes, it took place around 3 August and was developed further.

McClellan got told on the 1st of August (Halleck's message sent on the 30th of July) to test if the force in Richmond was "very small", and issued orders for a movement. It was delayed from the 2nd to the 3rd August because the guides got lost.
By the time McClellan's movement had taken place he'd already been ordered off the Peninsula, which appears to have been partly because Halleck didn't understand that the telegraph communication loop at the time was several days for a round-trip message (signal, transit, action, response transit, return).




Here it is from Lee's PoV, from Confederate Tide Rising:


The worst tidings of August 5 were yet to come, however. Even as Lee pondered his response to this new and unexpected threat from Fredericksburg, word arrived later the same night that McClellan had marched up the James with a large force and reoccupied Malvern Hill. To the Confederate commander, “it looked like” the long-dreaded “general advance of McClellan’s army” up the James had begun. 73

From all of the information available to him, Lee must have believed he now confronted the most critical moment of his brief career in the field. He scrambled to protect himself as best he could from these multiple threats. He ordered Hood’s division to move some five miles to the northwest to a point on the Brooke Turnpike, where it would be centrally located to march on either Ashland or Hanover Junction. 74 But it was toward McClellan— from whom he always expected the greatest danger— that he directed his full attention. He massed five divisions— his entire remaining force north of the James— at the foot of Malvern Hill, while he hurried to New Market to take personal charge of the defense against the Army of the Potomac. 75 He also ordered Anderson’s division at Drewry’s Bluff to be ready to march at a moment’s notice to join the main army north of the river. 76 And he wrote to D. H. Hill to “spare no effort in urging on the works at Drewry’s Bluff to a speedy conclusion,” as it might soon be necessary to order into the field the three brigades on loan from North Carolina. 77

When Lee arrived at Malvern Hill, he found “the enemy appeared in considerable strength. … His troops were drawn up in line of battle, his artillery in position, and he apparently was prepared to deliver battle in as strong force as he did” a month earlier. Because the day was “intensely hot” and the troops moved slowly, darkness fell before Lee had time to do more than advance a force east to Willis Church to threaten Federal communications with Harrison’s Landing. Whether Lee— remembering the bloody and futile assaults of July 1— was laying the groundwork for a turning movement, or whether he would have waited to develop McClellan’s intentions will never be known. The next morning, much to Lee’s surprise and confusion, the Army of the Potomac had disappeared, slipping back to its base at Harrison’s Landing.

It is clear that among his many speculations, Lee did not yet suspect that McClellan was actually preparing to withdraw the Army of the Potomac back to Washington, as had been ordered over the Federal commander’s protests four days earlier. Nor did Lee perceive there to be a strategic pattern uniting all of the Federal movements that would redound to the immense benefit of the Confederacy: He could not know that the hand of Henry Halleck, the new Federal general-in-chief, was already at work concentrating virtually all of the Union field forces in Virginia in front of Washington. Abiding by his own writings, which elevated concentration above maneuver, Halleck was preparing to duplicate his successful spring campaign against Corinth.

Ignorant of the true meaning of the enemy moves, it is also clear that Lee was shaken by the crisis. On August 7 he wrote to Jackson, “[ I] t seems at present… too hazardous to diminish the forces here”; Stonewall should plan to act without waiting for reenforcements. Lee then proceeded to critique a proposal Jackson had put forward in a letter of the 6th. 80 Apparently, Jackson believed the force at Fredericksburg was merely part of Pope’s army and not Burnside’s North Carolina contingent. Stonewall suggested that if he advanced against the Federal base at Warrenton he would draw all of Pope’s forces after him into Fauquier County and thus relieve the pressure on Richmond. Lee agreed the plan might work, if the Fredericksburg column were a detachment of Pope’s. But, if Pope had already concentrated his own army and Burnside were in fact at Fredericksburg with a whole new force, then Jackson would find himself confronted by a superior enemy in his front, while another dangerous foe operated on his right flank and rear. 81"







(McClellan moving back was because he'd been ordered to retreat, though it would take a considerable time to get all the wounded off. I also can't recall offhand what time McClellan actually got the retreat order, but the time delay on the earlier orders would suggest he got it some time on the 5th.)

A key point in the above is that Lee essentially was pinned at Richmond by McClellan's mere presence. He'd been able to detach Hood's division on top of the troops he sent away before August 5, because of the threat from Fredericksburg (where Burnside had landed), but if McClellan became the main threat he'd absolutely concentrate against him. (n.b. Confederate divisions were uneven in size and so "5 divisions" is a term which needs further examination.)
 
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Lee certainly beat McClellan in the first part of the Seven Days (i.e. up to Gaines Mill) which put McClellan in a position where he was forced to retreat. You can't really say McClellan "bottled it", though, because that implies his nerve failed - in fact if he hadn't retreated after Gaines Mill he'd have been surrounded and captured.
I am entirely open to hear what McClellan's alternatives were for the Seven Days sequence - want to suggest one? If it's off topic here, go here instead:

So should McClellan have put himself in this predicament? After all he's a strategic genius or was it the fact that Lee took away all his options by being the aggressor and dictating what McClellan would have to do.

There is no doubt in my mind that Lee was the puppet master and McClellan the puppet every time Lee pulled a string McClellan would respond by doing what Lee wanted.

I give McClellan some credit because of his over cautious nature and lack of aggression he kept casualties down and that's the reason why he was sacked not once but twice , Simply put he wasn't willing to let his beloved A0P become a butchers shop even if it meant not winning the war.

George McClellan chose the wrong profession.
 
So should McClellan have put himself in this predicament? After all he's a strategic genius or was it the fact that Lee took away all his options by being the aggressor and dictating what McClellan would have to do.
What, you mean the situation where his flank was open?

Well, he could have avoided this, but doing so would have violated a direct Presidential order (operate against the rail lines north of Richmond) and an order from the Secretary of War (fix your base on the Pamunkey river)
Alternatively he could just not have attacked Richmond indefinitely.

Which do you think he should have done?
 
I give McClellan some credit because of his over cautious nature and lack of aggression he kept casualties down and that's the reason why he was sacked not once but twice , Simply put he wasn't willing to let his beloved A0P become a butchers shop even if it meant not winning the war.
What are you talking about? McClellan's flank was open for the Seven Days because of his aggression, and he's the one who did almost all the attacking on America's Bloodiest Day. He certainly didn't attack to no purpose, but that's not generally considered a flaw...
As for why he was "sacked", do you mean the time he lost his job as GiC because he took the field with the army, the time his army was transferred from him to Pope and Pope pretty much immediately duffed it up (needing rescue by McClellan), or the time he was relieved of command entirely in November while about to attack Longstreet?

ED: if this is off topic, go over to the thread I have about what alternatives McClellan had. Please suggest an alternative McClellan had, and we can see how plausible it actually was...
 
So should McClellan have put himself in this predicament? After all he's a strategic genius or was it the fact that Lee took away all his options by being the aggressor and dictating what McClellan would have to do.

McClellan worked within the limitations Stanton and Lincoln set. Stanton specified that McClellan was not allowed to move his base from the Pamunkey to the James. Lincoln specified McClellan a secondary mission of cutting the rebel railroads north of Richmond to protect Washington. McClellan obeyed the lawful orders whilst knowing this created vulnerabilities.

By this time McClellan was already crossing the Chickahominy with the intent of cutting loose from the Pamunkey and heading for the James. His communications with the Navy show essentially he was aiming for Drewry's Bluff, because when the fortifications there were gone the navy could steam directly into Richmond.

Stanton and Lincoln crystallised the situation with their late May orders. McClellan was left with two options; hole up north of the Chickahominy and wait to be reinforced before advancing, or reinforce south of the Chickahominy and move in on Richmond when the rains stopped. He chose the latter. It was a calculated risk - if McDowell came down as promised the left bank was impregnable. If McDowell didn't, but held Jackson then the left bank was impregnable. McDowell didn't come, but Jackson did....

There is no doubt in my mind that Lee was the puppet master and McClellan the puppet every time Lee pulled a string McClellan would respond by doing what Lee wanted.

Then it's very odd that McClellan did what Lee considered to be the worst option for Lee every time....

George McClellan chose the wrong profession.

Hardly. As time showed, he was one of the best generals the US had. The problem was that the government and the public didn't want an efficient, modern general who knew his business and knew it well. They wanted something out of a pulp novel, who screams "Sparta" or "Freedom" and has zero clue about real operations.
 
It seems like the single greatest flaw in the historiography around McClellan is a persistent problem in evaluating firstly what a reasonable amount of force would be and secondly whether he had that force. This combines with the lack of understanding of the real reinforcement "schedule" McClellan got compared to what he was told he was getting, and leads to people describing him as "bleating" for reinforcements.

This is not a new thing. Lincoln completely lost track of the number of troops McClellan had been sent, which led to his quip about "shovelling fleas" and his question as to why McClellan had not advanced against Richmond with his 160,000 men. (The total number of men in McClellan's army, and the men absent, totalled to about 140,000.)
 
Saphroneth good info, lots happened that you really don't read about unless you get ahold of the right books Thanks for the education!
The funny thing about it all is that a lot of the information's actually right there in the ORs, though admittedly the force ratio issue requires more careful reading.
It's because the Confederates reported their strength, characteristically, as a low strength (e.g. "muskets" or "effectives") - "muskets" meant "all the non-officer ranked men who showed up to actually do the fighting job who were not skirmishers", while "effectives" meant "all the men, sometimes excluding officers, who showed up to actually do the fighting job". On ANV returns this ("effectives", though including officers) is what their PFD means - Johnston made the change.

Conversely the Union reported their strength as a high value, especially under McClellan (they reported PFD correctly by then-US Army regulations, meaning "all the men, officer or not, who showed up to their duty station that day, whether or not the duty station involved fighting").
The discrepancy could be significant, on the order of a 20% difference even when the force was not disrupted. It was quite possible for effective strength after hard marching and a day's hard fighting (e.g. September 18 at Antietam!) to produce a situation where effectives were half the official PFD or less.

There were four kinds of duty - normal (fighting), extra (basically logistics work) and special and daily (special was things like hospital orderly, daily was things like "cook").

Later on in the war the Union began shifting their definitions, and their PFD began to move closer to the Confederate definiton. Burnside introduced the PFDE category as the main strength measure (which excluded all the extra, special or daily duty men) and also moved extra duty out into "present" only. Since McClellan reported that it was common for about 1/6 of his army to be on extra duty, this hugely changed how things worked.

This means in turn that the Army of the Potomac's PFDE and the Army of Northern Virginia's PFD are close to the same measure after McClellan has left, but not while he's still in command (McClellan sometimes used PFDE but it was almost identical to his PFD). It's a problem which specifically strikes McClellan and means that his strength looks bigger than it was by the way almost any other commander he can be compared with (i.e. Johnston, Lee, Burnside, Hooker, Meade) measured it.


This is how it is that McClellan has 115,000 PFD for 127,000 Aggregate Present in June 1862 with his camp situated near a swamp and after about three weeks of solid rain, while Hooker has 133,000 PFDE for 158,000 Aggregate Present in April 1863. McClellan has 12,000 non-PFD, largely representing his sick, while Hooker has 25,000 non-PFDE in a healthier position. (McClellan on May 20 1862 has 102,600 PFD and less than 5,000 non-PFD.)

Similarly June 20 1863 the AotP has 79,000 PFDE but 108,000 Preset, and on June 30 it reports 84,000 PFDE on 118,000 Present - the non-PFDE is huge, representing almost a third of the army.

The diminuition from April 30 to June 20 in the Army of the Potomac's PFDE is enormous - about 50,000 men have gone, and they can't all be from Chancellorsville - and the drop in Present is equivalent. Some of it is due to reporting changes (the Cavalry Corps submittted no PFDE returns for June 20) but it looks like it might deserve more of a follow-up.
 
Wow just noticed I'm no longer a Cadet! You just explained a lot about all I've seen about the different numbers (Like Lee had less than 40,000-- no he had 70,000) I've seen for 1862 in Maryland. So bottom line, Fredericksburg on would be about the same for both sides.
 
Wow just noticed I'm no longer a Cadet! You just explained a lot about all I've seen about the different numbers (Like Lee had less than 40,000-- no he had 70,000) I've seen for 1862 in Maryland. So bottom line, Fredericksburg on would be about the same for both sides.
Roughly similar in reporting method, yes, though you always need to be careful to spot when errors can creep in.*
Interestingly it means we can use Burnside's returns to determine roughly what McClellan had on hand for the Loudoun Valley campaign, though not exactly because of the impact of extra weeks of winter campaigning on the effectives count (negative) and the addition of extra formations of troops (positive).



What makes it tricky is that even the people at the time weren't always clear on this, and you can state that someone had "70,000 troops" and "less than 40,000" and be correct in both cases... so long as you specify what the definition is. The Lost Cause statisticians played with exactly this trick, they always reported Confederate strength as low as possible and Union strength much higher to inflate Lee's accomplishments and fuel the "one Reb can whip three Yanks" idea.

Thus, the possible strength estimates for Lee at Richmond in July 1862 range from 200,000 (grand aggregate, literally everyone who has ever been sent to the army whether or not they've died/deserted, and including Jackson's formation) to about 56,000 (enlisted infantry effectives only with the army). They're all valid measures so long as you know what's being said.

Fortunately in 1862 most of the regiments haven't been consolidated/refilled much, so a good secondary proxy is regimental counts. At Antietam the number of infantry regiments for the sides is:

Lee - 185 including Thomas' Brigade.
McClellan - 186 including Couch's division.

McClellan's average regimental strength is somewhat higher because he has 17 brand new grass-green regiments in this total and because more of the damage to Confederate units in 1862 was sustained by units with Lee's army than of the damage to Union units in 1862 being sustained by units with McClellan's army, but functionally speaking McClellan's numerical advantage is small and largely consists of brand new regiments; for example, 12th Corps' first division has a brigade with three regiments from 1861 and three regiments averaging about a month old.



*An example of this is actually Burnside again, because his 9th Corps in 1864 had not changed reporting methods. So his list PFD with 9th Corps is very similar to AP, and his logistics troops need to be deducted.
 
Lee's army straggled badly on the Maryland invasion I see listed for the Stonewall brigade after 2nd Manassas 1,160 ( with the 2nd va having 196) at Sharpsburg no more than 250 men (less the 2nd va it is at Martinsburg) Was straggling that bad or is that much difference in counting heads. Must have been higher in the 2nd as a most the company's were from Jefferson, Clark, and Frederick county's.
 
Lee's army straggled badly on the Maryland invasion I see listed for the Stonewall brigade after 2nd Manassas 1,160 ( with the 2nd va having 196) at Sharpsburg no more than 250 men (less the 2nd va it is at Martinsburg) Was straggling that bad or is that much difference in counting heads. Must have been higher in the 2nd as a most the company's were from Jefferson, Clark, and Frederick county's.
It's a combination of several factors.

Firstly there's the straggling (as in "never reached the battlefield"), and in Maryland there was huge straggling simply because of the force of the marches. Ezra Carman suggested it was quite likely that hundreds to thousands of Confederates (from Longstreet/DH Hill/Walker/McLaws) were taken prisoner by being effectively run down on the march by the Union troops (having straggled and then not rejoined their units in time), and on top of that there were some troops who refused to cross the Potomac.
But secondly there's something much simpler - timing. Jackson's strength report was given on the 16th, less than an hour after he reached Sharpsburg, and at that time the majority of the division was still strung out on the road - Jackson's strength report for the division on the afternoon of the 16th, per Carman and Tom Clemens, is 1,784 infantry and 310 artillery, but the acting commander of Starkes' brigade stated that he fought (on the 17th) with 1,400-1,500.

Obviously it's not possible for an 18.5-regiment division to have 1,784 men if a 6.5-regiment component of that division is over 1,400 strong (the other 12 regiments would have to average 30 men each!), so what happened was that Jackson's command was measured just after they arrived but fought a day later once they'd closed up a lot. This would indicate that a good estimate of the four-regiment Stonewall Brigade on the field - less the 2nd VA which would have been regiment #5 - would be on the order of 900-1,000, depending whether you count officers. This would peg the regiments at about 200-250 each, which is more feasible than the 100 implied by the "1,784" number for the whole division and a whole lot more plausible than the "30" implied by that number and Starkes' Brigade.


We know from Meade that time has a huge influence on the effectiveness state of a unit. With an official (Union) PFD of about 15,000 and a theoretical combat strength of about 12,000 under ideal conditions, 1st Corps had only about 9,000 who fought on the 17th and maybe as low as 4,000 on the 18th, with the shattered 3rd Division mustering only about 300 effectives late on the 17th after the march and subsequent heavy casualties. The men were mostly still alive and would come in over the next few days, they were just scattered behind McClellan's army from exertion or finding 'perfectly reasonable excuses' not to come back and get shot at again today; that's a natural human reaction to danger we understand better now than they did back then, though I suspect McClellan had an inkling.
 
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As another illustration of how you need to dig a bit to find out what's going on with McClellan, there's the case at Yorktown. Much like with Antietam there are not complete records on what happened here for Magruder's force, but we know enough that digging gets us a rough estimate.


Firstly there's the conventional story, which is that McClellan arrived on the Peninsula with a very large army, advanced towards Yorktown, and got stopped by Magruder with a tiny force. Magruder's report (written some time later) states that he had only 11,000 men, 6,000 of them in Yorktown and at Gloucester Point, and that his remaining 5,000 men had to cover a length of 13 miles.

The problem, however, is that this is not an accurate description. Magruder's report also claims 200,000 men for McClellan (who at the time only had five divisions ready to move, meaning Magruder overestimated McClellan by about a factor of four) and the length of the part of the Warwick line that needed to be defended was only four miles. But that's not the big thing, and the big thing is actually a place where we can't simply rely on the ORs and we need to do research.


See, there was a late-April early-May report which listed just about every single infantry and artillery unit at Yorktown and gave their individual numerical strengths. This report was filed by Johnston, so we know it's going to be Effectives, and since it's from late in the siege it will not overestimate the strength of the force (for example, any effect from trench disease will already have been factored out).
This means the only thing we need in order to estimate the effectives at Yorktown with a high degree of accuracy is to know when every individual infantry and artillery unit arrived; after that we can just estimate the relatively scant cavalry, or ignore it with the understanding it's still actually there. And doing this analysis finds that on the 5th of April, when McClellan first reached the Warwick line, he was actually facing 14,600 infantry effectives and about 3,100 artillery effectives (part or all of the brigades of McLaws, Cobb, Ewell, Colston, Wilcox, Winston, Ward, Rains and Crump, though they weren't all assigned to those commanders yet) - the great majority of them occupying the whole line, rather than over half of them being crammed into Yorktown. Worse, the next day another 5,400 effectives had arrived (Cobb's Legion plus the two large brigades of Early and Rodes) bringing Magruder's infantry strength up to 20,000.



This is significant with respect to Antietam because it means that McClellan didn't "miss" an opportunity to attack an easy target - the Warwick line was a formidable position. This in turn fits with an understanding of McClellan not as a timid commander but a commander with a reasonable appreciation of risk/reward - he considered that he "had a chance" at Antietam that was worth trying, and while he failed he inflicted considerable casualties on top of winning the manoeuvre battle. In fact, arguably McClellan's error at Antietam would be attacking on the 17th instead of the 18th, as McClellan seems to have thought that not all Lee's army was "up" yet when only one division had not arrived; if McClellan had launched his attack on the 18th then almost his entire force would have been "up". (Smith, Slocum, Morell, Humphreys and Couch all arrived after the attacks began on the 17th; an extra day might also have let 1st Corps and the other earlier arrivals improve their effective strength.) On the other hand this may have allowed more Confederate digging-in, and McClellan had no way of confirming that the entire Confederate force was there - Couch was performing a screening role preventing Lee from turning McClellan's flank.
 
Interesting, I'd always thought McCellan's best move would be to attack Lee Longstreet's 3 divisions while Jackson's 6 were still at Harpers Ferry. McCellans troops had overwhelmed south mountain, I would have thought his troops were right behind Lee's.Then deal with Jackson, although but he would be hard to outmaneuver. McCellan probably didn't have up as many troops as I always thought.
 
Interesting, I'd always thought McCellan's best move would be to attack Lee Longstreet's 3 divisions while Jackson's 6 were still at Harpers Ferry.
Essentially the problem with that is the timing. McClellan's troops who were at the front of the column (at South Mountain) had marched all night and then fought all day (Lee pulled back at sunset) and Lee managed to make some distance overnight - it's much safer to march away from an enemy overnight than towards them, and Cox's corps seems to have been simply too tired to pursue vigourously. By the time Sykes and Richardson reached the Antietam on the 15th, Lee was the other side of it and fortifying.

With the distance from South Mountain to Antietam being a full day's march, and half of McClellan's vanguard (6th Corps) heading south to try and pin McLaws and Anderson between him and Harpers Ferry (stymied by the surrender), McClellan ended up with his first two divisions reaching Antietam Creek late on the 15th.

The build up by both sides goes:

Confederate
on the 15th: "Evans" division, DR Jones and DH Hill in place
by the 16th: Jackson, Ewell, and Walker arrive
by the 17th: Remainder of Jackson's column straggle in, plus McLaws and RH Anderson
During the 17th AP Hill arrives, plus more stragglers

Union:
On the 15th: Sykes and Richardson (2 divs) form line. Note that Sykes is an unusually small division.
On the 16th: The majority of McClellan's army arrives during the day, specifically 1st Corps, 2nd Corps and 12th Corps come "up" and 9th Corps is coming into position (12th and 9th would be able to arrive by nightfall if they were fed straight in, but any kind of finesse would take more time). This made seven divisions (Sykes is from 5th Corps) at noon with five (12th Corps has only two divisions) more arriving, but there were still five divisions nowhere near the battlefield.
On the 17th: 1st, 2nd, 12th and 9th are available, plus Sykes, and Morell and 6th Corps march in during the battle.

The question is really at what time an attack is considered plausible. On the 15th there's only two divisions actually present for McClellan, and arguably here Lee has the numerical advantage!
Fighting in the morning of the 16th is a bad idea, there's fog and most of McClellan's army was still marching to the field.
By noon on the 16th (when the fog burned off) the majority of Lee's army was closed up, with:
DH Hill
DR Jones
Hood/Evans
Jackson (straggled)
Ewell
Stuart
and Walker having collected stragglers crosses the Potomac at noon. Using the official strength for Jackson when he paraded on arrival, and assuming no return of Jackson's stragglers with Walker, this means that for an afternoon battle Lee has about 25,000 infantry effectives available to him (including Walker) and all of Stuart's cavalry, plus about 186 guns.

At noon McClellan had (in effectives, same as the above)
1st Corps: 8,600 infantry and 46 guns around the Pry House to the Pry (upper) Bridge
Richardson's division and Sykes' division: ca. 6,700 infantry and 30 guns at the Porter (middle) bridge
Artillery Reserve: ca. 42 guns, all the guns that engaged historically
Cavalry: ca. 2,500 and 22 guns

Sedgewick and French (the remainder of 2nd Corps) were at Keedysville, and could fight in the afternoon as they're close. This was about another 11,000 infantry and 30 guns (though Carman's numbers here are perhaps an overestimate).
12th Corps (ca. 7,200 infantry and 22 guns) was at Boonsboro, several miles away, and would only be able to come into action during the evening.
And 9th Corps (11,700 infantry and 32 guns, though again Carman's numbers here are a bit high) were about the same distance from the field as 12th.

This means that McClellan has the troops for an equal battle in the afternoon (about 10% more infantry, half the cavalry and slightly fewer guns) with an extra 19,000 to arrive by nightfall, though again this assumes 9th and 2nd Corps didn't straggle as much as the other Union formations. With Lee on the defensive it's a dangerous risk in the afternoon and a moderately risky attack in the evening, especially as the only way to get 9th Corps into action is to shove them across a bridge right at the enemy. (12th Corps can perhaps come into action in the evening by going across the Pry bridge that's controlled by 1st Corps, though they're unlikely to have time to do much.)

Using campaign strength instead (PFD no straggling) McClellan has 1/3 of 5th Corps, all of 1st Corps and all of 2nd Corps to hand, with 9th and 12th en route.
1st Corps 14,856
2nd Corps 18,813
One division of 5th Corps 6,450
Cavalry Division 4,320
44,439 Campaign Strength

9th Corps 13,819
12th Corps 10,126
23,945 Campaign Strength en route

while Lee has everyone except McLaws, Anderson and AP Hill, for about 50,000 campaign strength. (Via Thorpe: )
csArmyOct10-1862.jpg



This all suggests that an attack on the 16th before 9th and 12th corps had arrived would be a bad idea, because it's an attack against an enemy positioned for defence at equal odds.
 

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So I was thinking about what the outcome could have been if McClellan had had 6th Corps cross the Potomac and try and block it behind Lee, and to my mind the key issue is what Lee does in reaction.

Basically Lee has a resource allocation problem here. 6th Corps had about 9,000 combatants in the divisions of Smith and Slocum, so about 14,000 effectives total with Couch is a reasonable estimate after the marches.

Lee has several choices. One of them is for AP Hill to just basically hold the possible crossing point/s of the Potomac and block Franklin (which he should be able to do, AP Hill's division was one of the large ones and a river crossing like that is easy to hold) though this would rob him of AP Hill during the battle itself. This might come out a wash or even detrimental for McClellan.

If Lee had already ordered AP Hill to join him and Franklin gets over the Potomac (I'm not sure how plausible this is,
@67th Tigers ?) then Lee's options shrink. He can have AP Hill act as a rearguard by himself (which would do the same in terms of the impact on Lee's force as the previous option, but risk defeat much more for AP Hill - in campaign strength he'd be outnumbered about 2.5:1 and in effectives it's about the same) or add McLaws or Anderson to the rearguard, which would make it able to fight pretty much an even battle with Franklin in return for not having those divisions with him on the 17th.

Leaving both McLaws and Anderson to fight, along with AP Hill, means the serious risk of Franklin' defeat, but it also means that the fight at Antietam on the 17th is a likely Union victory.

And having Jackson's entire force from Harpers Ferry fight Franklin means Franklin's toast - but it also means the almost certain destruction of Longstreet's wing on the 17th.


My suspicion is that in fact crossing the Potomac there was not practicable after the Rebels left, but it's interesting seeing the risk/reward of what would happen if Franklin did cross. Functionally either Lee commits enough force south of the Potomac to fend off Franklin that the main fighting probably comes out a Union victory and Lee has to retreat to Boteler's Ford in disorder, or he doesn't commit enough force south of the Potomac and so Franklin ends up covering the southern side of Boteler's Ford in force - and Lee's army is screwed, as it's stuck unable to break out and surrounded by rivers.


It'd be an interesting alternate history scenario.
 
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