How many men does McClellan really think are in Lee’s army during the Maryland campaign?

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On September 11, 1862 near Rockville, Maryland, George McClellan writes to General-in-Chief Henry Halleck,

“All the evidence that has been accumulated from various sources since we left Washington goes to prove most conclusively that almost the entire rebel army in Virginia, amounting to not less than 120,000 men, is in the vicinity of Frederick City.”

On September 13 at Frederick, Maryland, McClellan finds Special Order 191, and, combined with his own intelligence, learns that the confederate forces are divided. Lee has moved the “main body of the army” to Boonsboro and Hagerstown, Maryland, and has sent several detachments south of the Potomac to capture 14,000 federal troops at Martinsburg and Harpers Ferry, Virginia.

According to Stephen W. Sears (George B. McClellan: The Young Napoleon, page 286), McClellan believes that Longstreet, Jackson, and D.H. Hill each command a corps of 30,000 men, while McLaws, Anderson, Walker, Hood, and A.P. Hill each command a division of 6,000 men. Order 191 states that Longstreet and D.H. Hill are at Boonsboro, while Jackson, McLaws, Anderson, and Walker are at Martinsburg/Harpers Ferry. Hood and A.P. Hill are not mentioned, so they could be anywhere.

If we do the math using these numbers, then McClellan believes that there are 60-72,000 confederates at Boonsboro/Hagerstown, and 48-60,000 at Martinsburg/Harpers Ferry.

McClellan decides to attack the confederates at Boonsboro in an attempt to quickly defeat Lee’s army while it is divided, but he also sends 20,000 troops under Franklin to rescue the men at Harpers Ferry.

With an army of only 95,000 men, why would McClellan detach Franklin’s 20,000 men?

First, McClellan would be left with only 75,000 men to attack up to 72,000 confederates at Boonsboro. That’s barely any advantage in manpower, and by attacking uphill through the mountain gaps he will have a significant terrain disadvantage.

Second, Franklin’s 20,000 men would be sent toward a confederate force of up to 60,000. Franklin would need to push through Crampton’s gap, then march down Pleasant Valley, and then attack up Maryland Heights. Does McClellan think that the confederates will neither defend Crampton’s gap nor move men across the Potomac River to Maryland Heights when they realize that the federals are threatening to lift the siege at Harpers Ferry?

I see no reasonable explanation for detaching Franklin except that McClellan doesn’t really believe that Lee has 120,000 men.

If McClellan thinks that Longstreet, Jackson, and D.H. Hill each command a corps, and an average corps has 3 divisions, and there are 5 additional divisions (McLaws, Anderson, Walker, Hood, and A.P. Hill), then this totals 14 confederate divisions in Lee’s army. If Stuart commands a division of cavalry, then it brings the total to 15. If each division averages about 6,000 men, then this means that Lee’s army has about 90,000 men. I believe that this is McClellan’s true estimate of confederate forces on the campaign.

Using this estimate, there should be 36-54,000 confederates (6-9 divisions) in each area. If McClellan thinks that he will face a maximum of 54,000 at Boonsboro, then it‘s understandable that he would be willing to part with Franklin’s 20,000 men, because with 75,000 he would still have at least a 21,000 man (40%) advantage against Longstreet/Hill.

90,000 is still a large overestimation of the size of the confederate army, but it’s not the ridiculous figure of 120,000 that he reports to Washington. After the Seven Days, he reported another grossly inflated number: 200,000 confederates around Richmond.

Such exaggerations serve his own interests. If he‘s up against superior forces and fails, then he has an excuse, like on the Peninsula, but if he succeeds, then he looks brilliant, like in West Virginia and Maryland.
 
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120,000 would be "aggregate present" using the standard internal conversion.

The estimates said 97,445 PFD, which appears to include GW Smith's Division, which was left at Richmond. It is a good and correct estimate:

Maryland%2Bestimate%2Bvs%2Breal.png
 
With an army of only 95,000 men, why would McClellan detach Franklin’s 20,000 men?
Among other reasons, the curtain of manoeuvre formed by South Mountain. There's only so many men that can attack at the gaps, and Franklin's men are sent to a different gap; McClellan also knew the Confederate army had been divided. Franklin's men were intended to pin a Confederate force between themselves and HF, and would have communications back to the main Union body because of the South Mountain curtain of manoeuvre.

In this sense the Union army remains cohesive because there's no way for a Confederate force to be driven into the gap between them.
 
Among other reasons, the curtain of manoeuvre formed by South Mountain. There's only so many men that can attack at the gaps, and Franklin's men are sent to a different gap; McClellan also knew the Confederate army had been divided. Franklin's men were intended to pin a Confederate force between themselves and HF, and would have communications back to the main Union body because of the South Mountain curtain of manoeuvre.

In this sense the Union army remains cohesive because there's no way for a Confederate force to be driven into the gap between them.
First, McClellan would be left with only 75,000 men to attack up to 72,000 confederates at Boonsboro. That’s barely any advantage in manpower, and by attacking uphill through the mountain gaps he will have a significant terrain disadvantage.

Second, Franklin’s 20,000 men would be sent toward a confederate force of up to 60,000. Franklin would need to push through Crampton’s gap, then march down Pleasant Valley, and then attack up Maryland Heights. Does McClellan think that the confederates will neither defend Crampton’s gap nor move men across the Potomac River to Maryland Heights when they realize that the federals are threatening to lift the siege at Harpers Ferry?

I see no reasonable explanation for detaching Franklin except that McClellan doesn’t really believe that Lee has 120,000 men.
 
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First, McClellan would be left with only 75,000 men to attack up to 72,000 confederates at Boonsboro. That’s barely any advantage in manpower, and by attacking uphill through the mountain gaps he will have a significant terrain disadvantage.
But there are only so many people who can even fit through the mountain gaps at all - historically there was a massive traffic jam.

Second, Franklin’s 20,000 men would be sent toward a confederate force of up to 60,000. Franklin would need to push through Crampton’s gap, then march down Pleasant Valley, and then attack up Maryland Heights. Does McClellan think that the confederates will neither defend Crampton’s gap nor move men across the Potomac River to Maryland Heights when they realize that the federals are threatening to lift the siege at Harpers Ferry?
What McClellan thinks is that there are Confederate troops around HF and that some of them are north of the Potomac with others south of it. The ones in the Pleasant Valley are more-or-less cut off from the ones south of the river, and are basically isolated; historically they had no way out until Harpers Ferry fell.
If the Confederates defend Cramptons Gap, then that's one thing, but it allows the prospect of a breakout from HF; if they don't, then Franklin is through and can trap the Pleasant Valley Confederates between himself and HF.


I see no reasonable explanation for detaching Franklin except that McClellan doesn’t really believe that Lee has 120,000 men.
Well, yes, because McClellan considered that Lee had ca. 100,000 men by the same measure by which McClellan had about 100,000 men.

The importance of the Pleasant Valley is that it constrains the forces which Franklin will actually encounter. There's these options:

1) The forces south of the Potomac are larger than the margin between Lee's army and McClellan's army (or rather, the forces north of the Potomac have McClellan outnumbering Lee), and the force in the Pleasant Valley is larger than Franklin's force.

In this case McClellan has reason to think he will win the fighting in the north, and Franklin's job is simply to fight a delaying action.

2) The forces north of the Potomac have McClellan outnumbering Lee but the force in the Pleasant Valley is small.

In this case Franklin has a good chance of success, and it's McClellan fighting in the north who is effectively fighting a delaying action. Once Franklin's succeeded then HF is relieved and can hold the crossing there, and Franklin can move north to join McClellan's main body.

3) The forces north of the Potomac have Lee outnumbering McClellan, but Lee has manoeuvred poorly and placed a force in the Pleasant Valley which is vulnerable.

In this case Franklin has a good chance of success, and the force south of the Potomac is too small to place Franklin in great risk. If he is threatened from the north he can fall back across South Mountain, but the action is worth a try.

4) The forces north of the Potomac have Lee outnumbering McClellan and the force in the Pleasant Valley (alone) is enough larger than Franklin's force that it's not vulnerable.

In this case the safer thing to do is to hold back and defend Washington.


Now, I happen to think that McClellan overestimated the strength of Lee's army by a comparatively small number (about 10,000-20,000 marching strength); this would place the total Confederate strength PFD at ca. 90,000 versus ca. 87,000 Federals actually available at the time (and more marching from the Washington area who historically arrived on the 17th and 18th).

Actual strengths in terms of bodies of men were:


Longstreet's unit was DR Jones and Hood/Evans, about 14,000 out of the 75,000 (post-Antietam plus casualty add-back) in Lee's force - about 20%.
Stuart's cavalry and DH Hill in the rearguard (about 16,000)
McLaws and Anderson just north of the Potomac, investing HF (about 18,500)
Jackson, Ewell, AP Hill, Walker south of the Potomac (about 27,500)

With McClellan putting ca. 20K PFD against McLaws and Anderson, and ca. 65K against Longstreet's main body, then...

Risk #1 - the total force north of the Potomac for Lee outnumbers McClellan.

With McClellan's estimate of enemy numbers being ca. 95,000, this is all but impossible unless there's practically nobody south of the Potomac. McClellan knows there are two separate enemy bodies moving south of the Potomac.

Risk #2 - Franklin is advancing against an enemy superior enough to simultaneously screen Cramptons Gap against him and also besiege Harpers Ferry.

With the width of Cramptons being such that it would take two divisions to hold, this is only possible if there's at least three divisions in the Mclaws/Anderson force (i.e. 25K or more). Franklin is not in serious danger if this is the case.

Risk #3: Franklin is advancing against an enemy strong enough to fight him off and put him in danger.

To endanger Franklin without uncovering HF would take at least 30K troops. In this case, and given that there are two separate enemy bodies moving south of the Potomac, McClellan would be able to assume that these two enemy bodies south of the Potomac had to total at least 20K troops between them. (One of them is Jackson's force, and Jackson's force is known to not be tiny.) This would put 50K or more troops around HF, which would leave less than 50K troops around Boonsboro.

Risk #4: The force at the northern gaps is strong enough that McClellan's main body (of 60K plus) can be stopped at the gap.

This is quite possible, but the nature of the gap means that it's an "even fight" - there's only so many troops both sides can fit into the gap.

Risk #5: The force at the northern gaps is strong enough that McClellan's main body can be endangered.

Not enough troops in McClellan's estimate of Lee's army.


Basically, the worst case scenario is that there are simultaneously enough troops at the northern gap to hold McClellan off (call it 30K), and enough troops at Cramptons Gap to endanger Franklin without giving up the HF siege (call it at least another 30K). If Franklin is having trouble and McClellan cannot break through the northern gaps then he can send more troops to reinforce Franklin - such as, say, Morell's division or one of the others in the middle of the marching order. (McClellan's army is a big traffic jam stretching back to Frederick, but moving a division or two out of the jam and pointing it at Cramptons actually frees up space on the road.)

This is not actually a big risk - there's no way for Lee to get around McClellan's flank or rear, even if this is "one big trap", because of the mountain geography. In fact McClellan's cavalry effectively screened his position and his attack through Cramptons provoked Lee to panic - Lee thought he was about to lose a quarter of his army in the Pleasant valley.


You may be asking however why it is that McClellan thought the enemy had about 120K, when I've been using a ~95K number for McClellan's upper limit of estimates and when the actual strength was about 75,000. It's simple - the 95K number and the 75K number are both PFD.

You convert PFD to Aggregate Present by multiplying by roughly 6/5. This raises 95K to about 114K AP (and 97.5K to near enough 120K); McClellan tended to use numbers like this for his planning because AP is less variable than PFD (and because, well, Lincoln liked to refer to McClellan's army in AP terms.)


Unfortunately the graphics in Thorp's Washington Post article which showed the Maryland Campaign situation in terms of known scouting reports (which would have given reason for McClellan to estimate the size of the force moving south of the Potomac etc.) are no longer available.



The bottom line here is that when comparing a Union army to a Confederate army you have to make sure you're using the same comparison. Both Effectives, both PFD, both AP, it doesn't matter so long as you're comparing both the same way.
 
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But there are only so many people who can even fit through the mountain gaps at all - historically there was a massive traffic jam.
Then why didn’t McClellan send more men with Franklin? Why did McClellan completely ignore Brownsville gap, 2 miles south of Crampton’s gap?
What McClellan thinks is that there are Confederate troops around HF and that some of them are north of the Potomac with others south of it. The ones in the Pleasant Valley are more-or-less cut off from the ones south of the river, and are basically isolated; historically they had no way out until Harpers Ferry fell.
If the Confederates defend Cramptons Gap, then that's one thing, but it allows the prospect of a breakout from HF; if they don't, then Franklin is through and can trap the Pleasant Valley Confederates between himself and HF.
There was a road connecting Maryland Heights to Sharpsburg.
6F4C7EFB-7741-44DA-BBB0-D13240606F84.jpeg
Well, yes, because McClellan considered that Lee had ca. 100,000 men by the same measure by which McClellan had about 100,000 men.
McClellan told Washington that he was facing 120,000 confederates in Maryland. By his counting, McClellan had about 95,000 men in his own army.
Now, I happen to think that McClellan overestimated the strength of Lee's army by a comparatively small number (about 10,000-20,000 marching strength); this would place the total Confederate strength PFD at ca. 90,000 versus ca. 87,000 Federals actually available at the time (and more marching from the Washington area who historically arrived on the 17th and 18th).

Actual strengths in terms of bodies of men were:


Longstreet's unit was DR Jones and Hood/Evans, about 14,000 out of the 75,000 (post-Antietam plus casualty add-back) in Lee's force - about 20%.
Stuart's cavalry and DH Hill in the rearguard (about 16,000)
McLaws and Anderson just north of the Potomac, investing HF (about 18,500)
Jackson, Ewell, AP Hill, Walker south of the Potomac (about 27,500)

With McClellan putting ca. 20K PFD against McLaws and Anderson, and ca. 65K against Longstreet's main body, then...

Risk #1 - the total force north of the Potomac for Lee outnumbers McClellan.

With McClellan's estimate of enemy numbers being ca. 95,000, this is all but impossible unless there's practically nobody south of the Potomac. McClellan knows there are two separate enemy bodies moving south of the Potomac.

Risk #2 - Franklin is advancing against an enemy superior enough to simultaneously screen Cramptons Gap against him and also besiege Harpers Ferry.

With the width of Cramptons being such that it would take two divisions to hold, this is only possible if there's at least three divisions in the Mclaws/Anderson force (i.e. 25K or more). Franklin is not in serious danger if this is the case.

Risk #3: Franklin is advancing against an enemy strong enough to fight him off and put him in danger.

To endanger Franklin without uncovering HF would take at least 30K troops. In this case, and given that there are two separate enemy bodies moving south of the Potomac, McClellan would be able to assume that these two enemy bodies south of the Potomac had to total at least 20K troops between them. (One of them is Jackson's force, and Jackson's force is known to not be tiny.) This would put 50K or more troops around HF, which would leave less than 50K troops around Boonsboro.

Risk #4: The force at the northern gaps is strong enough that McClellan's main body (of 60K plus) can be stopped at the gap.

This is quite possible, but the nature of the gap means that it's an "even fight" - there's only so many troops both sides can fit into the gap.

Risk #5: The force at the northern gaps is strong enough that McClellan's main body can be endangered.

Not enough troops in McClellan's estimate of Lee's army.


Basically, the worst case scenario is that there are simultaneously enough troops at the northern gap to hold McClellan off (call it 30K), and enough troops at Cramptons Gap to endanger Franklin without giving up the HF siege (call it at least another 30K). If Franklin is having trouble and McClellan cannot break through the northern gaps then he can send more troops to reinforce Franklin - such as, say, Morell's division or one of the others in the middle of the marching order. (McClellan's army is a big traffic jam stretching back to Frederick, but moving a division or two out of the jam and pointing it at Cramptons actually frees up space on the road.)

This is not actually a big risk - there's no way for Lee to get around McClellan's flank or rear, even if this is "one big trap", because of the mountain geography. In fact McClellan's cavalry effectively screened his position and his attack through Cramptons provoked Lee to panic - Lee thought he was about to lose a quarter of his army in the Pleasant valley.


You may be asking however why it is that McClellan thought the enemy had about 120K, when I've been using a ~95K number for McClellan's upper limit of estimates and when the actual strength was about 75,000. It's simple - the 95K number and the 75K number are both PFD.

You convert PFD to Aggregate Present by multiplying by roughly 6/5. This raises 95K to about 114K AP (and 97.5K to near enough 120K); McClellan tended to use numbers like this for his planning because AP is less variable than PFD (and because, well, Lincoln liked to refer to McClellan's army in AP terms.)


Unfortunately the graphics in Thorp's Washington Post article which showed the Maryland Campaign situation in terms of known scouting reports (which would have given reason for McClellan to estimate the size of the force moving south of the Potomac etc.) are no longer available.



The bottom line here is that when comparing a Union army to a Confederate army you have to make sure you're using the same comparison. Both Effectives, both PFD, both AP, it doesn't matter so long as you're comparing both the same way.
This is all based on an incorrect size of McClellan’s army. He has 95,000, with 7,000 more in Humphreys’ division way behind.
 
Then why didn’t McClellan send more men with Franklin? Why did McClellan completely ignore Brownsville gap, 2 miles south of Crampton’s gap?

Well, the answer to the first question is that Franklin didn't have any trouble getting through. If Franklin had had trouble then McClellan could have diverted more troops to Franklin, but Franklin didn't suffer enough of a holdup to make that necessary.

As to the second question, I can't actually find it on the map I have available to consult right now.
It looks like a very small gap, and if so it's no use for moving large bodies of troops and is for cavalry pickets to handle - or at most for Franklin's flank guard. Since Franklin did force Cramptons, it turned out to not be needed.



There was a road connecting Maryland Heights to Sharpsburg.
216131-6f5af4b3f44daeee67f75e55727f041c.jpg

It's a road which is very vulnerable to the HF garrison if Harpers Ferry holds out , which is why (per Harsh) Lee was ordering McLaws to try and get his men over Maryland Heights by any possible means, until HF surrendered.
You can't retreat 20K men down a narrow road, or if you try you'll have real trouble, and it'll mean abandoning all the transportation and so on; it'd also of course mean relieving HF.

This is all based on an incorrect size of McClellan’s army. He has 95,000, with 7,000 more in Humphreys’ division way behind.
95,000 by what measure? I was using PFD for both Lee's force and McClellan's there.
 
The numbers commonly given for Antietam (ca. 87,000 Federals, excluding Morell) are from the Aggregate Present of the army, as they marched out of Washington. Thanks to Scott Hartwig for pointing that out.

McClellan's moving force was approximately 94,000 Present, 78,000 PFD or ca. 61,000 combat effectives ca. the morning of 14th September.

Antietam%2Bagg%2Bpres.png


McClellan's intelligence said 100,000 PFD/ 120,000 Agg Pres for the enemy. This was the lowest estimate of anyone. Whilst trying to **** McClellan, Fishel gives a lot of of other estimates from others, most of which are much higher:

Stanton at the War Dept: 100,000 rebels at Frederick
New York Herald: 150,000
Curtin: 190,000 with Lee at Frederick, 150,000 more with Johnston waiting to cross at Leesburg and another 100,000 on the Rappahanock line
Pleasonton: 100,000
Banks: 150,000 just after Antietam, after receiving 15,000 reinforcements
Pinkerton: 97,445 PFD (rounded to 100,000 by McClellan who also quoted 120,000 which is the standard PFD to present correction he applied)
Wool gave two; 6-7 Sept: Jackson's corps has 30,000 and is at Frederick, and 8 Sept: Rebel army estimated at 75,000 and 30-50,000 seen by a scout sent to Frederick

In fact, if we assume McClellan believed his own estimates, which weren't that inaccurate, then he was attacking a moderately superior force. At Agg Pres, 95,000 vs 120,000 is a fair estimate of what he was being told. Yet he was confident enough to attack, whilst every other significant general was quaking in their boots, as was noted in England at the time.
 
Well, the answer to the first question is that Franklin didn't have any trouble getting through. If Franklin had had trouble then McClellan could have diverted more troops to Franklin, but Franklin didn't suffer enough of a holdup to make that necessary.
So he would rather bottleneck his army at Frederick, and then at Turner’s Gap and Fox’s Gap, than send more men with Franklin to Jefferson and then Crampton’s Gap?
As to the second question, I can't actually find it on the map I have available to consult right now.
It looks like a very small gap, and if so it's no use for moving large bodies of troops and is for cavalry pickets to handle - or at most for Franklin's flank guard.
The confederates thought that it was important enough to guard.
FA3FAD21-DC04-4250-BEE1-7A7944881527.jpeg

Since Franklin did force Cramptons, it turned out to not be needed.
It was needed because McClellan was supposed to be chasing Longstreet and Hill, and trying to cut them off before they reunited with the rest of Lee’s army. Instead, he decided to squeeze almost his whole army through Turner’s Gap and Fox’s Gap.
 
The numbers commonly given for Antietam (ca. 87,000 Federals, excluding Morell) are from the Aggregate Present of the army, as they marched out of Washington. Thanks to Scott Hartwig for pointing that out.

McClellan's moving force was approximately 94,000 Present, 78,000 PFD or ca. 61,000 combat effectives ca. the morning of 14th September.

View attachment 216244

McClellan's intelligence said 100,000 PFD/ 120,000 Agg Pres for the enemy. This was the lowest estimate of anyone. Whilst trying to **** McClellan, Fishel gives a lot of of other estimates from others, most of which are much higher:

Stanton at the War Dept: 100,000 rebels at Frederick
New York Herald: 150,000
Curtin: 190,000 with Lee at Frederick, 150,000 more with Johnston waiting to cross at Leesburg and another 100,000 on the Rappahanock line
Pleasonton: 100,000
Banks: 150,000 just after Antietam, after receiving 15,000 reinforcements
Pinkerton: 97,445 PFD (rounded to 100,000 by McClellan who also quoted 120,000 which is the standard PFD to present correction he applied)
Wool gave two; 6-7 Sept: Jackson's corps has 30,000 and is at Frederick, and 8 Sept: Rebel army estimated at 75,000 and 30-50,000 seen by a scout sent to Frederick

In fact, if we assume McClellan believed his own estimates, which weren't that inaccurate, then he was attacking a moderately superior force. At Agg Pres, 95,000 vs 120,000 is a fair estimate of what he was being told. Yet he was confident enough to attack, whilst every other significant general was quaking in their boots, as was noted in England at the time.

This is what you said in 2008:

The Confederate Army had about 35,000; organised thusly:

Lee's Reserve (reporting directly to Lee)
Anderson's Division (3,672 infantry, 16 guns)
Stuart's Cavalry Division (2,724 sabres, 4 guns)
Pendleton's Reserve Artillery (58 guns)
Evan's Independent Brigade (284 infantry, 6 guns)

Longstreet's Command (Right Wing/ 1st Corps)
DR Jones's Division (3,311 infantry, 4 guns)
Hood's Division (2,020 infantry, 24 guns)
Walton's Arty Bn (12 guns)

Jackson's Command (Left Wing/ 2nd Corps)
Jackson's Division (1,784 infantry, 21 guns)
Ewell's Division (3,904 infantry, 15 guns)
Light Division (2,231 infantry, 18 guns)

DH Hill's* Command (Centre Wing/ 3rd Corps)
DH Hill's Division (5,449 infantry, 24 guns)
McLaws' Division (2,231 infantry, 19 guns)
Walker's Division (3,764 infantry, 12 guns)

Which was correct? 95,000 or 35,000 - big difference.

https://www.google.com/url?sa=t&rct...engths.9547/&usg=AOvVaw1dvb5ZqMX3MEIeOekamZVu
 
We know exactly what McClellan thinks. In 1862 he's claiming upwards of 100,000 or 90,000. In his 1864 The Army of the Potomac, General McClellan's Report of its Operations While Under His Command he is blunt in saying that his estimation of 97,445 is accurate.

He says "An estimate of the forces under the command of Confederate General Lee made up by the direction of General Banks from information obtained by the examination of prisoners, spies, deserters, ect previous to the Battle of Antietam is as follows: ...These estimates give the actual number of men present and fit for duty."

That he believes there were upwards of 100,000 Confederates in the field is unquestionable, specifically there were 97,000. That these estimates are inaccurate is also unquestionable. McClellan got it wrong, plain and simple, but had to cling to that mistake to maintain his reputation.
 
Over the past few hours, I have done a little research on relative strength between AoP and ANV at Antietam. Here is the raw data of my admittedly brief research
Britishbattles.com

Numbers involved in the Battle of Antietam: The Federal Army comprised 87,164 troops with 275 guns. The Confederate Army comprised 41,000 troops with 194 guns.

Wikipedia

USA 87,164[4][5] CSA 38,000 "engaged

67thTigers blog March 19, 2016

Conclusions

RE Lee's AAR states "This great battle was fought by less than 40,000 men on our side, all of whom had undergone the greatest labors and hardships in the field and on the march. Nothing could surpass the determined valor with which they met the large army of the enemy, fully supplied and equipped, and the result reflects the highest credit on the officers and men engaged.". Lost Cause writers have tried to decrease this 40,000 figure further, but this doesn't seem reasonable (they further confuse matters by using different Federal measurements for comparison**). It is reasonable to state that Lee had around 40,000 effectives at Antietam.

Encyclopedia Britannica

Losses: Union, 2,108 dead, 9,540 wounded, 753 captured or missing of 87,000; Confederate, 1,546 dead, 7,752 wounded, 1,018 captured or missing of 45,000.

Essential Civil War Curriculum

Casualties in the battle of Antietam were horrific. The Army of the Potomac suffered 2,010 killed, 9,416 wounded, and 1,043 missing - 12,469 out of about 87,000 troops available. The Army of Northern Virginia lost 2,700 killed, 9,024 wounded, and 2,000 missing - 13,724 out of about 40,000 engaged.

US History .org

On September 15, Lee deployed his 30,000 soldiers on some four miles of rising ground behind Antietam Creek. He utilized the cover of rock outcroppings, rolling farmland, stone walls, fields of standing corn, and a sunken road in the center of his line.
Two days earlier, a Union corporal had found a copy of Lee's special orders wrapped around three cigars. But McClellan refused to act because he thought Lee's troops outnumbered his own. When McClellan started deploying his troops on September 16, he had 60,000 active soldiers and 15,000 in reserve. Had he thrust his complete force against the Confederates on September 15 or 16, he might have smashed Lee's army.

Note: Confederate total of 30,000 does not include A.P. Hill's Light Division which would bring the ANV total to about 35,000.


As you can see, the troop strength estimates are remarkably consistent with 35000-45000 for Lee and 75000-87000 for McClellan.
 
We know exactly what McClellan thinks. In 1862 he's claiming upwards of 100,000 or 90,000. In his 1864 The Army of the Potomac, General McClellan's Report of its Operations While Under His Command he is blunt in saying that his estimation of 97,445 is accurate.

He says "An estimate of the forces under the command of Confederate General Lee made up by the direction of General Banks from information obtained by the examination of prisoners, spies, deserters, ect previous to the Battle of Antietam is as follows: ...These estimates give the actual number of men present and fit for duty."

That he believes there were upwards of 100,000 Confederates in the field is unquestionable, specifically there were 97,000. That these estimates are inaccurate is also unquestionable. McClellan got it wrong, plain and simple, but had to cling to that mistake to maintain his reputation.
What McClellan said after the campaign is totally irrelevant. On the campaign, he tells Washington that he’s facing at least 120,000 confederates.

On September 11, 1862 near Rockville, Maryland, George McClellan writes to General-in-Chief Henry Halleck,

“All the evidence that has been accumulated from various sources since we left Washington goes to prove most conclusively that almost the entire rebel army in Virginia, amounting to not less than 120,000 men is in the vicinity of Frederick City.”
 
What McClellan said after the campaign is totally irrelevant. On the campaign, he tells Washington that he’s facing at least 120,000 confederates.

“All the evidence that has been accumulated from various sources since we left Washington goes to prove most conclusively that almost the entire rebel army in Virginia, amounting to not less than 120,000 men, is in the vicinity of Frederick City.”
And yet, they had at most 45,000 men total. How did the army's intelligence apparatus triple Lee's strength - or was it all in McClellan's mind?

Going through several books in my library over the years (Sears, Carman, Priest, McPherson, Martin) and the above brief internet search, I would estimate that the relative forces were 80,000 USA and 38-40,000 CSA. The confederate position at Antietam was extremely strong, but had no depth - a hard crust without much behind it. I have seen estimates that Lee had 400 guns in his artillery (mostly British sources). This is a wild exaggeration. There were probably only about 400 guns in both armies combined.
 
If you read the Official Records from the start of the campaign you can get a glimpse of how this occurred. Synopsis of reported intelligence estimates follow, omitting some duplicate, extraneous or fanciful sources (including one that Bragg with 40,000 men was moving down the Shenandoah Valley). BG Randolph Marcy was McClellan's chief of staff.
  • Sept. 4 - Pleasonton to Marcy - Stuart & Hampton at Dranesville (VA) with 15,000 cavalry.
  • Sept. 5 - Pleasonton to Marcy - Jackson, Longstreet, Smith & Hill with 30 - 45,000 moving toward Frederick.
  • Sept. 6 - Pleasonton to Marcy - A. P. Hill at Poolesville (MD) with 30,000.
  • Sept. 6 - Pleasonton to Marcy - Jackson with 30 - 40,000 near Barnesville (MD).
  • Sept. 6 - Pleasonton to Marcy - Jackson with 60,000 to move on Baltimore.
  • Sept. 6 - Sumner to Marcy - Jackson with 50,000 near Frederick.
  • Sept. 7 - Pleasonton to Marcy - 35,000 infantry & 5000 cavalry near Point of Rocks (MD).
  • Sept. 7 - Pleasonton to Marcy - 50,000 at Dranesville waiting orders to cross the Potomac.
  • Sept. 9 - McClellan to Halleck - Pleasanton's 9/8 report of 100,000 north of Potomac not reliable.
  • Sept. 9 - McClellan to Halleck - 110,000 near Frederick.
  • Sept. 9 - Thomas Scott (Pennsylvania RR) to McClellan - 100,000 near Frederick.
  • Sept. 10 - McClellan to Lincoln - Reports of Confederate strength range from 80 - 150,000.
  • Sept. 10 - Halleck to McClellan - 40,000 ready to cross near Monocacy.
  • Sept. 10 - Andrew Curtain (Governor of Pennsylvania) to McClellan - 120,000 near Frederick with more on the way.
As @Wesley P Ellington stated much of the information was from captured Confederates. Civilian observers with little or no experience in counting troops were also sources. Curtain's source was a minister relaying a report from a church elder who had passed thru Frederick. IMO one of McClellan's biggest failures as a commander was not setting up a competent intelligence service. That would await Joe Hooker in early 1863.

Edit: Corrected Pleasonton spelling.
 
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If you read the Official Records from the start of the campaign you can get a glimpse of how this occurred. Synopsis of reported intelligence estimates follow, omitting some duplicate, extraneous or fanciful sources (including one that Bragg with 40,000 men was moving down the Shenandoah Valley). BG Randolph Marcy was McClellan's chief of staff.
  • Sept. 4 - Pleasanton to Marcy - Stuart & Hampton at Dranesville (VA) with 15,000 cavalry.
  • Sept. 5 - Pleasanton to Marcy - Jackson, Longstreet, Smith & Hill with 30 - 45,000 moving toward Frederick.
  • Sept. 6 - Pleasanton to Marcy - A. P. Hill at Poolesville (MD) with 30,000.
  • Sept. 6 - Pleasanton to Marcy - Jackson with 30 - 40,000 near Barnesville (MD).
  • Sept. 6 - Pleasanton to Marcy - Jackson with 60,000 to move on Baltimore.
  • Sept. 6 - Sumner to Marcy - Jackson with 50,000 near Frederick.
  • Sept. 7 - Pleasanton to Marcy - 35,000 infantry & 5000 cavalry near Point of Rocks (MD).
  • Sept. 7 - Pleasanton to Marcy - 50,000 at Dranesville waiting orders to cross the Potomac.
  • Sept. 9 - McClellan to Halleck - Pleasanton's 9/8 report of 100,000 north of Potomac not reliable.
  • Sept. 9 - McClellan to Halleck - 110,000 near Frederick.
  • Sept. 9 - Thomas Scott (Pennsylvania RR) to McClellan - 100,000 near Frederick.
  • Sept. 10 - McClellan to Lincoln - Reports of Confederate strength range from 80 - 150,000.
  • Sept. 10 - Halleck to McClellan - 40,000 ready to cross near Monocacy.
  • Sept. 10 - Andrew Curtain (Governor of Pennsylvania) to McClellan - 120,000 near Frederick with more on the way.
As @Wesley P Ellington stated much of the information was from captured Confederates. Civilian observers with little or no experience in counting troops were also sources. Curtain's source was a minister relaying a report from a church elder who had passed thru Frederick. IMO one of McClellan's biggest failures as a commander was not setting up a competent intelligence service. That would await Joe Hooker in early 1863.
I believe it was probably similar to WWII pilots where three guys claimed the same kill and the intel people put it down as three separate kills. In Maryland, three people counted the same troops and the intel people put down the total rather than realizing that the three folks were in the same place at the same time.
 
I believe it was probably similar to WWII pilots where three guys claimed the same kill and the intel people put it down as three separate kills. In Maryland, three people counted the same troops and the intel people put down the total rather than realizing that the three folks were in the same place at the same time.
That was surely part of the situation. In defense of the Pleasonton and the other observers, they reported what they were told or thought they observed, leaving it to the boss to analyse. They usually identified their sources.
 
And yet, they had at most 45,000 men total. How did the army's intelligence apparatus triple Lee's strength - or was it all in McClellan's mind?

It didn't. Lee entered Maryland with roughly 75,000 PFD, which we can convert to ca. 90,000 present. In the absence of a early September return it can be approached three ways:

1. From as many returns for early September as are available in the CSR's and interpolate the rest. This was the methodology used by Martin & Busey for Gettysburg, and the late Prof. Joseph Harsh had a MA student, John Allen, do the same for this period. The summary is in 5H of Harsh's Sounding the Shallows, in the back-matter of Hartwig, and was copied into an excel sheet by Brett Schulte.

The result is 75,528 PFD.

2. From deduct forward from last known returns, minus known casualties from 2nd Bull Run. Livermore carried out this operation and calculated 77,135 PFD.

3. Add back casualties from Antietam and South Mountain from the first available complete post-campaign returns. Gene Thorp carried out this operation and calculated 77,769 PFD.

So we've three good estimates of what Lee crossed into Maryland with, one of 75,528, one of 77,135, and one of 77,769. All measured as PFD. We can thus be pretty sure, since we've approached the problem from three different directions and got the same result within a quite small variation, that this is correct.

These are PFD. Typically the accepted conversion (and the one McClellan's staff used)was x6/5ths to get aggregate present. This is close to the true value if you do a regression plot of every single available return.

So, Lee's army was a moving force of 90-92,000 white troops.

The next wrinkle is that Lee's wagon trains, and the regimental logistics (cooks etc.) are not included in these figures, being supplied by slave labour. Taking general observations and estimates, some 10-15,000 African-American slaves were traveling with this force. Meaning the whole force was roughly 100-105,000 men strong.

However, I predict you'll argue where were they all? Well in any army those who are sick, or performing other tasks (such as the logistics component) are simply not available for the line of battle. Deduct detachments and this moving force could not hope to place as many as 70,000 in line of battle. As Palfrey noted, reports of what was available for line of battle deducted the barefoot, logistics, and even the skirmishers were excluded. This would push this number lower. Of course, straggling in the forced marches to Sharpsburg would reduce this even lower. In fact the rebels had certainly less than 60,000 combatants on the field at Antietam, and not all of them at the beginning of the battle.

Grant later lamented attempts to try and distort the historical narrative by obfuscating the differences in how the two sides reported strength:

"At Shiloh the effective strength of the Union forces on the morning of the 6th was 33,000 men. Lew. Wallace brought 5,000 more after nightfall. Beauregard reported the enemy’s strength at 40,955. According to the custom of enumeration in the South, this number probably excluded every man enlisted as musician or detailed as guard or nurse, and all commissioned officers—everybody who did not carry a musket or serve a cannon. With us everybody in the field receiving pay from the government is counted. Excluding the troops who fled, panic-stricken, before they had fired a shot, there was not a time during the 6th when we had more than 25,000 men in line. On the 7th Buell brought 20,000 more. Of his remaining two divisions, Thomas’s did not reach the field during the engagement; Wood’s arrived before firing had ceased, but not in time to be of much service."

The rebel Lost Cause historians were trying to reduce the number of rebels at Shiloh to 20-25,000, exactly as they tried to reduce the numbers at Antietam to 40,000. For some reason, unfathomable to me because surely no-one would be biased, some historians accepted their arguments regarding Antietam (and the Seven Days), but rejected them for Shiloh, and are have recently been busy dismantling them for the Overland Campaign.

In answer to your question; no, they were not in McClellan's mind, or anyone's mind. Intelligence did not triple Lee's strength. In fact the estimates are really pretty good.
 
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