Measuring Lee's army in Maryland
There are these ways in which we can measure the strength of Lee's army.
What Steiner Saw
Other observations
Taking the September 2 strength and go forwards from there
Taking the October 10 report (the first complete report, which includes the cavalry) and adding back either admitted Confederate casualties, total derived casualties (based on burials, captures and known wounded) or a casualty count based on the second but reduced according to how many were still sick
Taking the September 22 report (which is noted as "very imperfect") and adding back casualties (either admitted or derived), for an estimate of troops with their units on the field
Taking Carman's strengths on the field (at Antietam) and going through carefully to tease out errors
Generalized company/regiment count
Officer count by analogy with non-new regiments from McClellan's force
Not all of these strength measures will generate the same number, and some will or will not account for straggling. The September 2 report (the Schulte ORBAT) is in Effectives, and the same or similar is true for the September 22 and October 10 reports (which are in Confederate PFD).
Method | What does it show | What should we compare it with |
Steiner’s observation | Size of the moving armed bodies, not counting wagons | Not immediately clear – Union PFD or Union AP |
Other observations | Size of the moving armed bodies | Probably Union PFD or AP, depending on if wagons are included |
September 2 strength | Confederate Effectives before straggling | Union effectives calculated as 0.8 of PFD (known calculation) |
October 10 strength and add-back | Confederate PFD for the campaign | Union PFD with casualties added back, with caveats |
September 22 report and add-back | Confederate strength on the field (plus Thomas’ brigade) | Union PFD on the field, from McClellan’s report, though with caveats |
Carman’s strength | Effectives on the field at Antietam | Carman’s strength for the Union |
Regiments of all arms | Estimated strength based on ROAA | Union ROAA count (green regiments are stronger numerically but not necessarily in combat terms, and heavily depleted Union regiments partly compensate) |
Officer count | Strength in any given category | Union strength in the appropriate category (i.e. compare officer strengths) |
What Steiner saw:
Steiner saw the main body of the Confederate force (Jackson's corps, Longstreet's corps, McLaws, Anderson, and the reserve artillery) to be "not more than 64,000".
This is:
557 companies in Longstreet's command
141 companies in Anderson's division
683 companies in Jackson's command
164 companies in McLaws' command
19 companies in Pendleton's reserve artillery
1,564 total
And implies 40.9 men per company in this category.
He also saw DH Hill's division (minus GB Anderson) to be 8,000. This is 193 companies, and implies 41.5 men per company in this category.
The other observations have been compiled by Gene Thorp for an article in the Washington Post, and they are:
Observation | Number | Companies | Men per coy |
Whole army, September 4-6, said by a Virginia captain in Leesburg | 84000 | 2015 | 41.7 |
Whole army, September 8 | 75000 | 2015 | 37.2 |
Whole army, September 9, Rebel deserter | 100000 | 2015 | 49.6 |
Whole army, September 9, telegraph operator, over 100,000 | 100000 | 2015 | 49.6 |
Whole army, September 10, newspaper, 60,000 to 100,000 | 80000 | 2015 | 39.7 |
Main Body (Jackson Longstreet McLaws Anderson res art), September 10 | 60000 | 1564 | 38.4 |
Main Body (Jackson Longstreet McLaws Anderson res art), September 10 | 70000 | 1564 | 44.8 |
Main Body (Jackson Longstreet McLaws Anderson), September 10, “40,000 to 60,000” | 50000 | 1564 | 32.0 |
Walker | 6000 | 92 | 65.2 |
Jackson, Williamsport, “15,000 men and 70 pieces of artillery” | 15000 | 683 | 22.0 |
Jackson, south of Williamsport, “not less than 25,000” | 25000 | 683 | 36.6 |
Jackson, 6 miles west of Williamsport, “15,000 infantry, 100 cavalry, 40 pieces of artillery” | 15000 | 683 | 22.0 |
Jackson, west of Martinsburg, “with a force of 20,000” | 20000 | 683 | 29.3 |
McLaws and Anderson, “30,000 men at Burkittsville” | 30000 | 305 | 98.4 |
McLaws and Anderson, “25,000 down to 12,000” | 18500 | 305 | 60.7 |
Anderson, “8,000 or 10,000 of the enemy” | 9000 | 141 | 63.8 |
McLaws and Anderson, “some 30,000 of the enemy’s troops on the heights” | 30000 | 305 | 98.4 |
McLaws or McLaws and Anderson, “I thought there were some 12,000 or 15,000, perhaps more” | 13500 | 305 | 44.3 |
Longstreet’s Command, “20,000 to 30,000” | 25000 | 557 | 44.9 |
McLaws and Anderson passing over from Maryland Heights, counter gave up after counting 17,600 infantry in 22 regiments averaging about 800 | 17600 | 305 | 57.7 |
McLaws and Anderson passing over from Maryland Heights, “20,000, perhaps 30,000” | 25000 | 305 | 82.0 |
Total attacking force at Harpers Ferry (Jackson, McLaws, Anderson, Walker) “amounting in all to 40,000” | 40000 | 1080 | 37.0 |
September 2 strength
The Schulte ORBAT has the total strength of the Army of Northern Virginia on September 2 as 75,687 men (in effectives), which equates to a little over 94,000 Union PFD given the conversion factor Schulte used when generating Union strength for this report.
In major formations:
Unit Name | Strength | Cos. | Effectives per coy | PFD per coy |
Confederate Army: September 2, 1862 | | | | |
Army of Northern Virginia / General Robert E. Lee | 75687 | 2015 | 37.6 | 47.0 |
Longstreet's Command / MG James Longstreet | 19624 | 557 | 35.2 | 44.0 |
Jones' Division / BG D.R. Jones | 3724 | 126 | 29.6 | 36.9 |
Wilcox's Division / BG Cadmus Wilcox | 5582 | 130 | 42.9 | 53.7 |
Kemper's Division / BG James L. Kemper | 4887 | 152 | 32.2 | 40.2 |
Hood's Division / BG John B. Hood | 3839 | 92 | 41.7 | 52.2 |
Evans' Independent Brigade / BG N.G. Evans | 996 | 49 | 20.3 | 25.4 |
Longstreet's Artillery Reserve | 590 | 8 | 73.8 | 92.2 |
Anderson's Division / MG R.H. Anderson | 5712 | 141 | 40.5 | 50.6 |
Jackson's Command / MG Thomas J. "Stonewall" Jackson | 20608 | 683 | 30.2 | 37.7 |
Jackson's Division / BG W.E. Starke | 5646 | 198 | 28.5 | 35.6 |
Ewell's Division / BG A.R. Lawton | 6383 | 218 | 29.3 | 36.6 |
A.P. Hill's "Light" Division / MG A.P. Hill | 8570 | 267 | 32.1 | 40.1 |
Cavalry Division / MG J.E.B. Stuart | 5664 | 127 | 44.6 | 55.7 |
Reinforcing Column | 24071 | 507 | 47.5 | 59.3 |
McLaws' Division / MG Lafayette McLaws | 7765 | 164 | 47.3 | 59.2 |
D.H. Hill's Division / MG D.H. Hill | 9848 | 232 | 42.4 | 53.1 |
Walker's Division / BG John G. Walker | 5159 | 92 | 56.1 | 70.1 |
Reserve Artillery / BG W.N. Pendleton | 1299 | 19 | 68.4 | 85.5 |
McClellan's force marching out of Washington (discounting cavalry) was on the order of 178 regiments (1,780 companies) and was on the order of 75,000 PFD, which gives about 42 men per company. This is on par with Longstreet's force, less than the Confederate reinforcing column and more than Jackson's hard-fought formation, which is a good sign for this calculation being reasonable. The accession of reinforcement regiments over the course of the campaign would tend to drive up the average per-company strength (though he did not join, Humphreys for example was about 6,800 in 82 companies and would thus be an average of 83 men per company organization) towards parity with Lee's total.
I should also point out that the Confederate batteries are often quite strong, though I've counted them as one company each. This may be the result of them just not being in the forefront of the fighting much - no battle yet had had the Confederate artillery line "overrun", especially not the reserve artillery which is broken out separately here. At Second Bull Run meanwhile many Union batteries lost all their guns.
To add back casualties, we need to know how many casualties were suffered in the campaign as a whole (this will generate numbers which are whole-campaign, not Antietam).
We have the following known information:
McClellan's medical director, Letterman, reported 2,500 wounded Confederates under his care following the battle.
OR S1 V19 P1 Pg 111:
"In addition to our own wounded, we had upon our hands from the battles of South Mountain, Crampton's Gap, and Antietam in all about 2,500 Confederate wounded".
At about the same time, about 3,000 unwounded prisoners were held in Fort Delaware (of which 3/4 were exchanged) and a minimum of 224 were in Fort McHenry (that's the number exchanged from there).
In the cemeteries associated with Antietam there are 3,270 Confederate graves, of which 690 were buried in Shepherdstown and Winchester (and thus were buried by the Confederates) and the rest of which were buried or reburied by the Union:
The Confederates who were captured mortally wounded and subsequently perished in Union hospitals were buried at Mount Olivet, and numbered 112.
The Confederates reinterred at Washington Confederate Ceremony years after being buried on the battlefield numbered 2,468.
The number of Confederates who were mortally wounded and died at Shepherdstown numbered 114 (and were buried at Elmwood).
The number who died at the hospitals in Winchester and were buried there is 576.
There are a number of disconnected reports which give Confederate casualties. One report gives the casualties in troops under Longstreet's command as 964 KIA, 5,236 WIA and 1,310 MIA.
Another for Hood's division alone gives casualties from Northern Virginia and Maryland campaigns mostly intermingled, and if the Maryland campaign casualties alone are used gives 122 KIA, 807 WIA and 87 MIA.
The most complete single report is Lafayette Guild's report of killed and wounded at South Mountain, Crampton's Gap, Harper's Ferry, Antietam and Shepherdstown. This report however misses out some regiments and even entire brigades (such as the cavalry and the reserve artillery).
It gives as the total 1,567 killed and 8,724 wounded. The shortcomings of this report are:
AP Hill's division is listed en masse rather than broken out into regiments.
Rodes' brigade is listed en masse.
Jones' brigade (3 regiments, 1 battalion) is not listed.
The following regiments are not listed:
4th SC Bn
3rd SC Bn
Philips' Legion
9th VA
5th FL
61st VA
10th VA
1st Louisiana Zouaves
26th GA
38th GA
58th VA
14th LA
21st GA
21st NC
1st NC Sharpshooters
5th NC
12th NC
13th AL
In addition some batteries are not listed (though it is possible they took no casualties) and the cavalry, as mentioned, is unlisted.
All told this means that Guild's report misses more than 10% of the Confederate infantry formations and all the cavalry.
Totalling the more granular individual reports (as done by Gene Thorp, looking at OR Vol. 27, pages, 843, 861, 862, 888, 925, 974, 975, 983 and 1026) comes to a total of 1,674 killed, 9,451 wounded and 2,292 missing for a total of 13,417. This may also be missing some casualties.
To tally this up without double counting, we would begin:
MIA
5,000 or more (as this is the number of wounded Confederates under Letterman's care, plus the number of unwounded Confederates who were held in Forts Delaware and McHenry and were subsequently exchanged. Using 5,000 excludes about 750 men who were not exchanged but for these purposes I will be doing this).
KIA
2,468 (as this is the number who were buried on the field of battle)
As of yet there is no double counting.
Moving on to Confederate reports, the only way to be certain we are avoiding double counting is to count only the total listed wounded in the Confederate reports. If the reports were compiled immediately after the army crossed the river, then using the wounded count and adding burials on the Confederate side of the river would risk double counting men who died in hospitals after the report was compiled. (Anyone who was left behind wounded would either die - and be buried on the field - or not die - and become one of the captured wounded statistics.)
This number is 9,451 wounded.
The total estimate of Confederate casualties in the Maryland Campaign thus comes to a lower bar of 16,919.
I should note here that post-war the surgeon-general assessed the Confederate casualties of the campaign as:
Harpers Ferry
500 WIA
South Mountain
500 KIA, 2343 WIA, 1500 MIA
Antietam
3500 KIA, 16399 WIA, 6000 MIA
Shepherdstown
33 KIA, 231 WIA
Total
4033 KIA, 19473 WIA, 7500 MIA
So the 16,919 number is a lower bar and the casualties could have been significantly higher, which would in turn push the estimates of things like the amount of straggling that took place lower.
Post battle reported strength and add-back
The September 22 return, the first filed after the battle, gives:
Longstreet's corps 1,574 officers and 19,001 men (total 20,575)
Jackson's corps 1,158 officers and 14,685 men (total 15,843)
These are given in PFD.
This report is noted as being "very imperfect". It does not include the cavalry or reserve artillery.
The September 30 report gives:
Longstreet's corps 2,018 officers and 26,481 men (total 28,499)
Jackson's corps 1,784 officers and 21,728 men (total 23,512)
The reserve artillery is listed as 55 officers and 716 men (total 771)
The cavalry is still not listed.
This report is also noted as being "incorrect".
The October 10 report gives:
Longstreet's corps 2,146 officers and 27,934 men (total 30,080)
Jackson's corps 1,917 officers and 25,603 men (total 27,520)
Cavalry 423 officers and 5,338 men (total 5,761)
Reserve artillery 54 officers and 858 men (total 912)
At this date Lafayette Guild reported that there were still about 5,000 sick and wounded at Winchester and that they were being moved to Staunton.
Given that the number of men who died of wounds south of the Potomac is 690, this allows us to estimate that of the 9,451 wounded we initially had on record as south of the Potomac then as of October 10:
690 died
3,761 may have returned to the ranks*
5,000 remain convalescent
* This estimate assumes that no wounded were sent to anywhere else or kept at Shepherdstown, and is therefore the largest possible estimate.
Adding back casualties in the same way that McClellan did to estimate his pre-Antietam strength, and bearing in mind that Confederate-recorded PFD may not be the same as Union-recorded PFD:
Using the September 22 report, but the 30 September number for artillery and the 10 October number for cavalry:
Longstreet 20,575
Jackson 15,843
Reserve artillery 771
Cavalry 5,761
All casualties 16,919 (this is a slight overestimate as some of the 3,761 casualties estimated to have recovered by October 10 may have been cavalry or reserve artillery)
Total estimate 59,869
This is effectively the Confederate strength in the campaign, minus stragglers (particularly stragglers south of the Potomac, such as on the marches undertaken by the commands of Jackson, McLaws, Anderson and Walker). It also includes some of the post-Antietam disruption to the Army of Northern Virginia.
Using the October 10 report, and counting 3,761 casualties as having returned to the ranks:
Longstreet's 30,080
Jackson 27,520
Cavalry 5,761
Reserve artillery 912
All casualties 16,919
-3,761 returned to the ranks
Total estimate 77,431
This is effectively an estimate of the strength with which Lee campaigned during the Maryland Campaign. It is quite close to the 75,000 figure from Allen's MA thesis (which is Effectives).
The next one on the list is Carman's strengths. This doesn't mean Carman's raw numbers, but going through to (1) identify the cases where Carman used numbers which may be or are not quite in the right category and (2) include unengaged units, which is something Carman did not do.
Doing this at Antietam for the Confederacy results in:
DH Hill: DH Hill claimed 3,000 infantry, but Carman calculated 5,449 infantry and 346 artillery.
Via Clemens we get:
Ripley's brigade: 1,349 infantry into action
Rodes' brigade: 3rd, 6th and 12th Alabama carried 840 into action, excluding at least the officers of the 12th. 5th Alabama was a small regiment, but no clue about the 26th; if the 5th and 26th average the same as the other three then it's ca. 1,400.
Colquitt's brigade, from Carman's correspondence:
6th Georgia: 300+ (and one company on picket not counted)
23rd Georgia: 485 men (not officers)
27th Georgia: 400 men (not officers)
28th Georgia: 250 men or 250-275 officers and men
13th Alabama: Carman guessed 310 based on a 2nd September strength of 612
Which implies that there were ~1,780 men plus officers, for a brigade strength of ~1,900
Garland's brigade:
Carman gives the brigade 756, but it was the strongest in the division on the 2nd September (though it did then take plenty of casualties at South Mountain) and the 5th North Carolina had ~625 men at Antietam. Adding officers and assuming the other four regiments are each about half the size of the 5th NC gives an estimate of about 2,100, but I'd say we could go with 1,700 to avoid overclaiming. Effectively this is assuming the other four regiments average 250 officers and men.
GB Anderson's brigade - Carman gives 1,174 and this seems fairly solid.
Divisional total:
1349+1400+1900+1700+1174 = 7,523 infantry, and 346 gunners.
McLaws: Carman gives 2,823 infantry and 138 gunners. This might actually make sense, on 2nd September McLaws disposed of 7,340 infantry and 419 gunners but they got hit hard at Crampton's Gap and suffered 1,907 claimed infantry casualties. If the actual casualties were higher and there was the usual straggling, or those casualties and a bit more than the usual straggling, then I suppose it's possible they could be down to about 3,000.
RH Anderson: The brigades except for Armistead (who remained in reserve) were sent to reinforce DH Hill, who estimated it at ca. 3,000-4,000. As we've seen DH Hill's estimated strength was really low for his own division, so it could be considerably higher. (The 22nd September return gives 5,324 PFD, and it suffered 1,278 casualties at Antietam; it'd probably be reasonable to say it had about 5,000 actually with the division once Armistead is counted.)
Walker: Carman calculated 3,764 infantry and 230 artillery.
DR Jones: DR Jones himself estimated "only 2,430 men" from a pre-campaign PFD of 8,953 infantry; he suffered about 1,000 casualties in the pre-Antietam fighting, but being down to less than 2,500 simply isn't credible. In particular this division includes Drayton's brigade, which is the one that had had almost no straggling until September 11th, so a lot of the hypothetical stragglers would have had to fall out of line and be swept up.
Carman estimated 3,311 infantry and 81 artillery, but going brigade by brigade:
Toombs: Toombs claimed over 400 muskets in the 2nd and 20th Georgia, while Carman estimated 357 officers and men in the same two regiments. Then 15th and 17th Georgia (Carman est. 281 muskets) joined them, and half the 11th Georgia (reported as 140 muskets in the five companies present).
This implies about 750-780 men minus officers, or a bit more with officers (so ca. 825).
Garnett: Carman gave 261 officers and men in the brigade, arguing that the brigade was recruited from near where Lee crossed the Potomac and briefly deserted. This would imply something like 80% of the brigade had either done that or was elsewhere.
Kemper: Carman estimates 443 on the field by adding regimental reports and guessing for the 28th VA, but others reported the brigade at 500-600 muskets (which would imply 600 or so with officers added, indicating the 28th VA was a strong regiment). 600 would also indicate straggling of about 50%.
GT Anderson - Carman estimates 597, but see Hood/Evans...
Drayton: Carman estimates 465, excluding 11th Georgia (detached and one wing fought in Toombs' brigade, 140 men counted there). Carman was told by a veteran the 15th SC had 300 men, but he used 200 in his calculation. Carman's estimate of 100 for the 51st Georgia is given no basis but we'll use it. The Phillips Legion infantry was also present, but Carman doesn't include it.
Correcting the 15th SC from 200 to 300, and adding the Phillip's Legion (ca. 135) plus adding officers would be ca. 700 men plus about 80 officers, so 780.
Jenkins: Carman quotes a newspaper to give 755 but in correspondence with the brigadier he's told 1,250 officers and men plus the 1st SC (106). So 1,356.
This totals to 825+261+600+780+1356 infantry, for 3,822 infantry (not including GT Anderson) plus 81 artillery.
Hood/Evans:
Carman says Evans' brigade straggled really badly, down to 284 infantry and 112 gunners for 396 all told. That's down from 1,058 on 2nd September, though there were 216 casualties so it means "only" about 53% straggling, or 47% of the strength in line.
Hood's brigade and Whiting's brigade are estimated at 2000 infantry (Hood gave this) and 304 artillery, and this does not include officers (so ca. 230 officers). 2230 infantry means about 62% of theoretical strength in line.
GT Anderson's brigade: 597, and Evans claimed it in his report. That's about 43% of theoretical strength which is another bad straggle.
Total for these four brigades considered here is 3,111 infantry and 416 artillery.
Jackson: As covered above, the number Carman gives is 1,784 infantry and 310 artillery, but this is just after the arrival on the field. If we take Starke's number of 1,400-1,500, take the low number (1,400) and correct the other three brigades to match then we multiply their September 2 strength by 0.86, to give:
Grigsby 1,000
Johnson 700
Warren 1,330
Starke 1,400
Total 4,430 infantry and 310 artillery
Ewell:
Carman gives 3,904 infantry and 223 artillery, but for once there's full reports in the OR. Ewell moved more slowly than the Stonewall division and took care to keep his straggling under control (he paused south of Boteler's Ford to collect up stragglers before crossing), and Lawton and Early's brigades match very closely to 22nd September PFD plus casualties; the other two don't.
It looks like there's an artillery battery buried in the reports for Hays and Trimble (as Ewell's division reported artillery in brigade totals) and the numbers for Hay and Trimble are also a bit vague - Hays' entire brigade is reported as the force Hays took to the wheatfield, and Trimble is just "out of less than 700 men carried into action". Assuming the four brigades behaved similarly you get about 1,000 in both (1078 instead of 550 for Hays and 1029 instead of 700 for Walker) but this includes the artillery. This would give a total of about 4,400 infantry and 223 artillery, and would mean that there were 73% of the infantry in battle - what we'd expect from a division that controlled straggling especially well.
AP Hill: Carman gives 2,231 infantry for the three engaged brigades, plus 337 artillery. The three engaged brigades were Branch, Archer and Gregg (2nd September total 3,887, so 57.3% of 2nd September strength engaged), with Pender and Field (2nd September total 2,932) not engaged; if Pender and Field straggled as much as Branch, Archer and Gregg, that gives an extra 1,683 infantry for a total of 3,914 infantry and 337 artillery on the field.
Thomas was of course not on the field.
Total strengths of the infantry divisions:
DH Hill 7523 + 346
McLaws 2823 + 138
Anderson 5,000 + 0
Walker 3,764 + 230
DR Jones 3,822 + 81
Hood/Evans 3,311 + 416
Jackson/Stonewall 4,430 + 310
Ewell 4,400 + 223
AP Hill 3,914 + 337
Total infantry 38,987
Carman gives the cavalry directly at 4,500, and the total artillery at 3,629. This would mean a grand total of
47,116 effectives
This estimate has large error bars on it, and could be off by a few thousand either way. There were also quite a lot of men who have to have been just behind the lines, as taking the September 22 return and adding back casualties gives just under 60,000 for the whole army - a difference of 13,000 men.
The comparable number for the Union is:
Infantry
1st Corps
Meade said that ca. 9,000 were carried into battle, and Carman estimated 8,619 infantry were on the field. Notably they got only 63% of their infantry PFD into the firing line as assessed at the time.
2nd Corps
Carman gives 4,029 for Richardson
5,437 for Sedgewick
5,740 for French
But these numbers are dubious.
67th Tigers goes through the whole corps in a post:
http://67thtigers.blogspot.com/2009/08/strength-of-2nd-corps-at-antietam.html
And the estimate for effectives given by this analysis is on the order of 12,500.
4th Corps:
Not at Antietam itself on the 17th.
5th Corps
Carman estimates:
1,640 for Buchanan's brigade, but no supporting source can be found
The rest were not engaged.
It would be reasonable to say:
1,060 PFD for Lovell's brigade (not effectives),implying about 800 effectives
5th and 10th NY: unknown, but but small - the 5th NY history gives effective strength and the 10th at the same ratio would give them just 225. I'll use 500 however (25 per company).
(Sykes on September 2 in the Schulte ORBAT is 2,938 infantry, this gives them 2,940 which seems reasonable)
Morell: 5,407 PFD for the whole division, including artillery (two batteries of which were in Carman's count of engaged). This would indicate about 4,000 effectives, but the given PFD is lower than expected pre-straggling PFD by a couple of thousand (but then again Morell's division had marched all the way from Arlington to Antietam in five days, which would tend to induce straggling). I'll use 5,000 effectives as a broad figure, giving the whole of 5th Corps as just under 8,000 infantry effectives.
Humphreys was not at Antietam on the 17th.
6th Corps:
Franklin: Frankin said he had 8,100 effectives at South Mountain, and deducting 533 South Mountain casualties and assuming none of that figure is artillery gives 7,467 infantry effectives. This is an overestimate.
9th Corps
Carman gives 11,714, but several of the regimental strengths he uses are PFD. Again 67th has looked at this:
http://67thtigers.blogspot.com/2009/08/strength-of-9th-corps-at-antietam.html
And the rough number he has reached is about 8,400 infantry effectives.
12th Corps
Carman gives 7,239. This implies that 71% of PFD is infantry effectives, which suggests that it is high.
Artillery: Carman gives 5,982 engaged, and there was one battery of Morell which was not engaged in Carman's definition. Using 118 for ease of calculation gives 6,100.
Cavalry: Carman includes units off the field, but going through by companies estimates 2,636 cavalry on the field.
This gives a total of
52225 infantry
2636 cavalry
6100 artillery
For 61,000 effectives on the field.
If all corps got 62% of their PFD into line as infantry effectives the total strength on the field would be 58,499. This gives the general picture of the Union army being roughly 24%-30% stronger on the field than the Confederate one, though much of this would be in "green" regiments (there were 21 "green" regiments on the field at Antietam, inclusive of Morell but exclusive of Humphreys, and these would represent roughly 11,070 effectives at 62% of an average PFD of 850 - most or all of the difference in the army sizes)
Union corps | PFD (pre straggle) at Antietam | Inf eff at Antietam | % of PFD as inf eff | Notes |
1st | 13996 | 8619 | 61.58% | This is the one we have the best information for |
2nd | 17953 | 12500 | 69.63% | This one might still be high |
5th | 12930 | 8000 | 61.87% | |
6th | 11440 | 7467 | 65.27% | Known overestimate |
9th | 13819 | 8400 | 60.79% | |
12th | 10126 | 7239 | 71.49% | This one might still be high |
| | | | |
Green regiments | 17850 | 11067 | 62.00% | 62% by definition here |
It's worth noting at this point that the Union PFD on the field on the 17th was about 83,000.
I've already been largely alluding to it, but the next category is regiments of all arms. The figure for the Confederates is 2,015 companies (201.5 regiments of all arms), though only about 197 ROAA were at Antietam, and for the Union it is:
Corps | Division | Inf regiments | Arty batteries | Cav coys | Total coys | Notes |
Army | Escorts etc | 1.6 | | 11 | 27 | |
1st | Doubleday | 17 | 4 | | 174 | |
1st | Ricketts | 12 | 2 | | 122 | |
1st | Meade | 11.7 | 3 | | 120 | 4 PA R has 5 coys, 13 PA R has 6, 2 PA R has 8, 10 and 12 PA R has 9 |
1st | Escorts | | | 4 | 4 | |
2nd | Richardson | 13.8 | 2 | | 140 | 69th NY and 52nd NY have 9 coys each |
2nd | Sedgwick | 13.8 | 2 | | 140 | 1st MN has 11 coys, 72nd PA has 15, plus 2 coys of sharpshooters |
2nd | French | 10 | | | 100 | |
2nd | Escorts | | 3 | 2 | 5 | |
4th | Couch (not present 17th) | 15 | 4 | | 154 | Includes 139th PA, which joined 17th |
5th | Sykes | 8.8 | 3 | | 91 | 68 total regular coys, plus Warren |
5th | Morell | 20.2 | 3 | | 205 | Extra coys of MI and MA sharpshooters |
5th | Humphreys (not present 17th) | 8 | 2 | | 82 | |
5th | Escorts etc | | 7 | 2 | 9 | |
6th | Slocum | 11.8 | 4 | | 122 | 18th NY is 8 coys |
6th | Smith | 14.4 | 3 | | 147 | 43rd NY is 5 coys, 3rd VT is 9 |
6th | Escorts | | | 2 | 2 | |
9th | Wilcox | 8 | 2 | | 82 | |
9th | Sturgis | 8 | 2 | | 82 | |
9th | Rodman | 7 | 1 | | 71 | |
9th | Kanawha | 6 | 2 | 1 | 63 | |
9th | Escorts etc | | 2 | 10 | 12 | Includes 6th NY (8 coys) |
12th | Williams | 11.1 | | | 111 | Zouaves d’afrique just 1 coy |
12th | Greene | 11.2 | | | 112 | 28th PA is 15 coys, but Purnell Legion/3 MD/109 PA are all 9 |
12th | Escorts etc | | 7 | 1 | 8 | |
| Averell | | | 31 | 31 | 10 coys 5th US, 4th PA, 9 coys 6th PA |
| Pleasonton | | | 36 | 36 | 8th IL, 3rd IN, 1st MA |
| Horse Art | | 3.5 | | 3.5 | Half a battery with McReynolds |
| Scattered other coys | | | 20 | 20 | 3rd PA, bn 12th PA, bn 1st US |
| McReynolds (not present 17th) | | 0.5 | 32 | 32.5 | |
For 231 regiments of all arms, of which 204 were on the field at some point on the 17th.
The final category is officers.
Number of Confederate officers on the reports:
September 22 (very imperfect): 2,732 (12.2 ORs per officer)
September 30: 3,857 (12.6 ORs per officer)
October 10:
4,540 (13.2 ORs per officer)
4,117 without the cav div (13.2 ORs per officer)
Union officers on the returns:
1st Corps September 18:
374 (16 ORs per officer)
1st Corps September 22:
666 (22 ORs per officer)
October 10 return (first with officers broken out)
5139 officers in the field (20.3 ORs per officer)
This supports the argument that the Confederate strength category "PFD" is not the same as the Union. If the average Confederate division had as many ORs per officer as the Union 1st Corps on the 22nd then the Confederate strength on the 22nd would be expected to be 46,444 PFD (officers and men) on that date (which is after casualties).
Conclusion
The
exact strength on the field is hard to define, but there are these common threads:
The Union and Confederate army strengths were quite similar, and somewhat discounting the new green regiments means we can largely see the two armies as comparable in combat strength in Maryland (and indeed at Antietam itself). Any significant disparity in strength would result from what happens during the campaign (i.e. straggling).
Lee's decision to invade the North now actually makes sense. He had beaten an army significantly larger than his own at Second Bull Run, and subsequently been reinforced; he had no reason to think that in a "fair fight" he couldn't produce a win.
There is no data to support the idea that the Union force outnumbered the Confederates 2:1 by any consistent measure.