Decoding the Union Army at Antietam

[Waremblem]

For a battle as well documented as Antietam there is a surprising amount of discrepancy toward the actual numbers engaged on both sides. Leaving aside the Rebels for the moment, I'm diving into the Union Army trying to get an accurate count. Often you will hear the number 87,164 thrown around for the Army of the Potomic. However, I think this is the on paper strength at the start of the campaign pre-South Mountain. A break down looks like:

First Corps - 14,856
Second Corps - 18,800
Fifth Corps - 12,900
Sixth Corps - 12,300
Ninth Corps - 13,800
Twelth - 10,000
Cavalry - 4300

Total - 86,956

I rounded some of the Corps after 1st, so that gives us the oft cited total of 87,164. However, this was on paper strength, probably compiled by McClellan in the reorganization when he took over. Mac was an excellent administrator so let's assume this was actual strength having already removed ineffectives from the roles. Many people forget the army fought a fairly significant battle at South Mountain before Antietam. Union casualties are generally listed at 2000-3000 here although there would certainly be stragglers, especially since the Army of the Potomic was moving at a fairly rapid pace to counter Lee's invasion. David Eicher, certainly comprehensive in his detail, credits the Union with 75,500 at Antietam. This matches what Stephen Sears in perhaps the definitive book on the battle, Landscape Turned Red comes up with, 75,000. However, Sears says the effective strength was 71,500 (probably deducting the missing division of Humphreys from V Corps). It is interesting to note that for those looking to run a full Antietam campaign the amount of attrition the Army of the Potomic suffered between one winning engagement at South Mountain and the battle at Antietam.

Effective forces for the Union at Antietam appear to be:

First Corps - 8600 (!)
Second Corps - 15,100
Fifth Corps (deducting Humphreys) - 10,300
Sixth Corps - 12,000
Ninth Corps - 12,500
Twelfth Corps - 7200
Cavalry - 4300

Total - 70,000

Now it's unlikely the cavalry suffered no casualties so the number likely drops under 70K. I'm rather astonished at the losses suffered by 1st Corps prior to the battle and that really changes the narrative doesn't it? For the longest time we heard the story of how Hooker's powerful First was stopped by the numerically inferior rebels. But it seems Jackson had around 7700 men which means Hooker barely outnumbered him while bearing the onus of the attack. In fact, if you add in the 12th Corps which assisted Hooker, you only come up to the original operational strength of First corps at the onset of the campaign. Using these numbers, when wargaming Antietam it may not be quite the "sure-thing" some critics of McClellan have made it out to be. In fact, table top generals might come to the same realization Mac did - he didn't have to win the battle of Antietam, but he could not lose it.

[DaveH]

Interesting analysis - I've found that trying to really work out the numbers engaged in ACW battles can be complex given the OR numbers tend to be obtained away from time of the battle so are probably higher than reality.

[Westmarcher]

Coincidentally, I was looking at my copy of Fitzhugh Lee's biography of his uncle, Robert E. Lee, today. He states that Lee was unable to ride his horse that day due to an earlier riding accident when leaving Virginia and I wished to be sure of this before inserting this comment on an American Battlefield Trust video on YouTube. While looking for that source I came across a reference to numbers as follows:-

• Fitzhugh Lee maintains that Lee's army at Sharpsburg totalled 35,255.
• That Lee told Fitzhugh he fought the battle with 35,000 troops
• McClellan reports he had in action, on the 17th, 87,164 troops of all arms

but further on in the book states that

• the fighting on the Federal side was done by four corps, numbering 57,614 men with Porter's and Franklin's Corps and the cavalry numbering 29,550 troops not engaged.

Other than Lee, I don't know what his sources were but if this reduces the Union army to about 58,000, this lends further support to your theory that victory for McLellan was not so much the "sure thing" some critics have made it out to be.

[Waremblem]

I have McClellan making his attack with 43,400 in three separate actions - 1st and 12th toward the Dunker Church (15,800), 2nd toward the Sunken Road (15,100) and the 9th at Burnside's famous bridge (13,800). Franklin and the 6th (12,000) was eventually partially engaged and there is some contention here on whether their use could have been decisive in breaking the Confederate center had they been committed earlier. McClellan is partially to blame here as his HQ was too far from the front to act decisively - although battles are tricky things. Lee gets credit for sending Hood in to save Jackson but that appears to have been mostly luck with Hood asking Jackson that morning if he could retire from the front lines after heavy skirmishing the night before with his men needing ammunition and food. Further, Hood's First Texas bridage covered itself with glory achieving far more than anyone could have reasonably expected.

McClellan never committed the 5th Corps, and it is quite reminecent of Napeoleon with the Guard at Borodino. He appears to have considered unleashing the 5th to achieve decisve victory late in the day, but the Corps commander, Porter is famously quoted as saying "Remember General, I command the last reserve of the last army of the Republic." Here again context must be addressed. The Union army under Pope had just been destroyed at Second Manassas. As I initially mentioned, McClellan could not lose his army and there is good reason to believe McClellan already felt the rebels were beaten. Which in fact, they were. Antietam is an enormous strategic victory for the Union stopping the invasion and allowing Lincoln to issue the Emancipation Proclamation (freeing all slaves). In fact, the Union captured 13 guns, took 6000 prisoners and 39 colors during South Mountain and Antietam while not losing a single flag or gun. By these standards, McClellan did win a decisive victory, although he did not destroy the Rebel army.

I don't think McClellan ever had great numerical superiority in the battle, especially considering he was attacking. His biggest two mistakes, were not committing Franklin at the decisive point and perhaps not using Porters fresh 5th Corps to pursue and break up the retreating Confederates. However, that is Monday Morning Quarterbacking. It is unfortunate McClellan died before he could write down his account. Perhaps he felt the Rebels were broken and there was no need to pursue. It could be argued that Lincoln exasperated the situation by replacing Mac with the incompetent Burnside who got himself mauled at Fredricksburg, a mistake McClellan would almost certainly have avoided. This defeat squandered much of the strategic initiative won at Antietam. Finally, one must remember that scholarship for the longest time tended to embellish Lee. So much in the way Aspern-Essling was spun as a "draw" for Napoleon so too was Antietam for Lee. This has not withstood modern scrutiny and it is perhaps time we give McClellan his due for winning the bloodiest single day battle of the ACW.