Brigade / Division / Corps Sizes

[tony_gunter]

i notice in many battles, sizes of troops groupings are quite different.

Many Confederate brigades were nearly rhe size of a Federal Division. Confederate divisions were somewhere between a division and a corps. And when battles were large enough to match a Confederate corps vs a Federal corps such as at Chancellorsville, the Confederates enjoyed overwhelming numerical superiority.

What was the origin of the difference in sizes for brigade, division, and corps, and how did this affect the performance of the two sides?

 

[Rhea Cole]

The building blocks of Civil War chains of command were regiments. On paper a regiment was made up of 1,000 men. So, two regiments = a brigade, four brigades = a division, three divisions = a corps plus artillery & quartermasters. (+/-) 10,000 -12,000 men is the book solution strength of a corps.

After the retreat from Nashville the Army of Tennessee rallied south of the Tennessee River. A Tennessee regiment that was the consolidated remnants of three regiments had 40 men who answered the roll call. Only twenty something of them were armed.

There are your brackets, 1,000 to 40 men from three regiments, by the book is the equilivant to a brigade.

A rule of thumb that Civil War historians I know use is a regiment = 350 men (+/-). It is useful to recall the image of a major university marching band, (+/-) 300-350 members. Deployed in line of battle the footprint of a regiment was (+/-) a football field.

There are so many variables in any specific scenario that there is no hard & fast strength that applies across the board.

Do, the answer to the question you posed is… it depends.

 

[Stryker65]

i notice in many battles, sizes of troops groupings are quite different.

Many Confederate brigades were nearly rhe size of a Federal Division. Confederate divisions were somewhere between a division and a corps. And when battles were large enough to match a Confederate corps vs a Federal corps such as at Chancellorsville, the Confederates enjoyed overwhelming numerical superiority.

What was the origin of the difference in sizes for brigade, division, and corps, and how did this affect the performance of the two sides?

At Chancellorsville, Jackson's Corps had four divisions of five brigades each. That's twenty brigades. The biggest Union corps had three divisions of three brigades each. At the time, Lee only had two infantry corps, so his manpower was only divided into two. Hooker's infantry manpower was divided into seven parts. Additionally, Confederate units in Virginia were always understrength, which made it necessary to combine regiments in brigades. Typically, Union brigades had -- at the most -- five (sometimes six) regiments. Confederate brigades at least had six, at least in the Western Theater, while in the Eastern, ANV brigades had usually four. Four regiments! That means that 4-5 Confederate brigades equals 3 Union brigades. On paper, Confedearte forces have more units, but that was typically not the case. Twenty Confederate brigades versus nine Union, in the case of Chancellorsville's Jackson vs Howard, would become only about a 5-3 ratio, if not less.

An example for Union structure is the reorganization of the Union XX and XXI Corps. Both were nine-brigade corps. For some awful, logistical reason, the War Dept. ordered brigade consolidations like this:

in which every new IV Corps brigade consisted of two or more others consolidated. Needless to say, the orders of battle are horrifying. At Chattanooga, every IV Corps brigade had nine or more regiments, 21,000 men present. Before the consolidation (a week after Chickamauga), XX and XXI Corps combined contained a total of only 17,000. Compare this, though, to the strength of Hancock's Second Corps a week before the Wilderness: 28,000 men in eleven brigades. Compared to II Corps, IV Corps was normal.
Here are the numbers of XX and XXI Corps on 9/28/63:

And here are the numbers of IV Corps on 10/31/63:

.
After the battle of Chickamauga, the XX Corps and XXI Corps each numbered only divisional-size. Number-wise, it would make sense to consolidate them. The main point here, however, is that the Confederate Army had no problem with consolidating units (such as 42nd/46th/48th/49th/53rd/55th Tennessee). The Union Army never consolidated units. Thus, in the orders of battle, we see nine regiments in a brigade. This brigade would have ordinary brigade strength.

In conclusion:

tldr see the bolded above

 

[rpkennedy]

The Union Army never consolidated units.

I wouldn't go quite that far. While they were resistant to the practice, it did happen just not on the level as seen in the Confederate armies.

Ryan

 

[Stryker65]

I wouldn't go quite that far. While they were resistant to the practice, it did happen just not on the level as seen in the Confederate armies.

Ryan

True, but most instances were due to mustering out. Only rarely were Union units consolidated due to losses; most of the time the remnants of the unit would become "attached" to another unit until replacements could fill it up again.

 

[Jamieva]

There's no standardization in either side as to how many divisions go in a corps, or brigades in a division. That is some of the confusion as to how a brigade could fight a division or a division could fight an entire corps. AP Hill had a 5 brigade division but other divisions in Lee's army only had 2 or 3 brigades.

On the Union side the XI and XII corps were the equivalent of a confederate division. They each only had 6 brigades total in each corps.

The Union finally got into the consolidation mode with corps in 1864 and jettisoning the smaller corps and merging them into the larger ones. For example the I and III corps go away and get assimilated into the II, V, and VI corps for the start of the Overland.

 

[Stryker65]

Here's the full pdf from my AoC consolidation work:

Quite a lot of 2-brigades-become-1...

 

[LCYingling3rd]

It was always difficult for me to understand as well. I had an ancestor in the 2nd VA Infantry of the five regiment "Stonewall" brigade. After the fighting at Spotsylvania Court House though, the 2nd VA was placed in Terry's Brigade of Gordon's Division with fourteen regiments! That is one giant brigade! However, it was simply that three brigades had been so decimated in the fighting their reduced sized regiments were all combined into that single brigade.

And then there was my ancestor that joined the 7th Virginia Cavalry. At the beginning of the conflict, it grew so large it was brigade strength by itself! So many wanted to fight with the flamboyant Turner Ashby, "The Knight of the Valley," that the 7th became known as "Ashby's Cavalry!" It was eventually broken into smaller, regimental sized units with the 7th VA becoming part of the Laurel Brigade.

On the Union side, however, my great grandfather was in the 87th PA Infantry. He had joined the unit in October of 1862. The regiment had formed a year earlier. During Sheridan's 1864 Valley Campaign the 87th was in the first brigade, third division of the 6th Corps. In October, the "veterans" of the 87th, who had joined in "61, got in line, saluted and marched off to York, PA where they mustered out. My great grandfather was left with barely two hundred other soldiers in the regiment. They were designated the 87th PA Battalion and fought on with the brigade. Daniel Reigle, fighting with the reduced 87th PA battalion at Cedar Creek, even won a medal of honor!

It took me a while, however, I did learn that the "Regiment being ten companies of a hundred soldiers each" rarely lasted long in the field. With casualties, disease, furlough, and desertion regiments were lucky to muster five hundred in their ranks. The fact is that you must determine unit strengths battle by battle and that can be very difficult to do.

 

[Stryker65]

Heavy artillery regiments, having a maximum strength of 1,800 men, were often divided up similarly to Ashby's Cavalry. The 2nd Pa. H. A., for instance, had so many recruits that two regiments were formed out of it: the 2nd PAHA and the 2nd Provisional PAHA. Each was split into battalions, as each battalion numbered about the size of a field regiment or two at that time. By the end of 1864, both had lost so many men that the Provisional one was consolidated back into the original one.

 

[Jamieva]

It was always difficult for me to understand as well. I had an ancestor in the 2nd VA Infantry of the five regiment "Stonewall" brigade. After the fighting at Spotsylvania Court House though, the 2nd VA was placed in Terry's Brigade of Gordon's Division with fourteen regiments! That is one giant brigade! However, it was simply that three brigades had been so decimated in the fighting their reduced sized regiments were all combined into that single brigade.

And then there was my ancestor that joined the 7th Virginia Cavalry. At the beginning of the conflict, it grew so large it was brigade strength by itself! So many wanted to fight with the flamboyant Turner Ashby, "The Knight of the Valley," that the 7th became known as "Ashby's Cavalry!" It was eventually broken into smaller, regimental sized units with the 7th VA becoming part of the Laurel Brigade.

On the Union side, however, my great grandfather was in the 87th PA Infantry. He had joined the unit in October of 1862. The regiment had formed a year earlier. During Sheridan's 1864 Valley Campaign the 87th was in the first brigade, third division of the 6th Corps. In October, the "veterans" of the 87th, who had joined in "61, got in line, saluted and marched off to York, PA where they mustered out. My great grandfather was left with barely two hundred other soldiers in the regiment. They were designated the 87th PA Battalion and fought on with the brigade. Daniel Reigle, fighting with the reduced 87th PA battalion at Cedar Creek, even won a medal of honor!

It took me a while, however, I did learn that the "Regiment being ten companies of a hundred soldiers each" rarely lasted long in the field. With casualties, disease, furlough, and desertion regiments were lucky to muster five hundred in their ranks. The fact is that you must determine unit strengths battle by battle and that can be very difficult to do.

After Spotsylvania those 14 regiments were all decimated that is why they were consolidated. The Stonewall brigade got wiped out at the Bloody Angle and basically ceased to exist.

 

[67th Tigers]

The numbers of Corps in both armies were regulated by the government, and a Corps could only be created, or disbanded by their respective Presidents.

In the Federal Army this meant Army Corps kept getting weaker. The main field force of the Army of the Potomac was created with 12 divisions, each of ca. 10,000 men organised into four Army Corps (i.e. ca. 120,000 men). One of those divisions would be ordered away (Blenker) and eventually would have an extra bde added and be redesignated an Army Corps (12th), and one would be detached leaving 3 Army Corps with 8 divisions of ca. 10,000. A division was transferred from 1st Corps, and a temporary reorganisation into five Corps was made, with an Army Corps now being two divisions, and a Corps being 20,000.

Due to combat and a lack of replacements, the Army Corps strength drifted downwards until by Gettysburg each Corps was the strength of a division, and it bottomed out at about 8,000 men per Corps in late summer. During this period, each division was weaker than an establishment brigade.

The minimum strength for an Army Corps to really function as a Corps is about 20,000, and it needs it's own cavalry as well as artillery. This was achieved in this period by pairing up Corps into Wings or "Grand Divisions," with all three arms. The reality was at Gettysburg the Federals had three operational "Army Corps," but each was composed of 2 or 3 Corps, and a division of cavalry.

The sensible thing to be done would have been to amalgamate Corps, and this was finally done, with 1st and 3rd Corps disbanded. However, commanders cannot order this. Army Corps were created by the President, and could only be disbanded by the President.

 

[LCYingling3rd]

After Spotsylvania those 14 regiments were all decimated that is why they were consolidated. The Stonewall brigade got wiped out at the Bloody Angle and basically ceased to exist.

Yes. I have gone to Spotsylvania where the weather worn trenches at the Mule Shoe are still very well preserved. It was very moving to stand where my ancestor of the 2nd VA was and imagine the terrific fighting there. It is mind-boggling to imagine how any of them survived as wave after wave of Union soldiers poured over the breastworks into the mud and joined in the hand-to-hand fighting. It must have been brutal!

 

[Saphroneth]

Much of the way things were done in the Civil War was based on the example of the Napoleonic Wars. In the Napoleonic Wars, a corps was an army in miniature - with not just infantry, cavalry and artillery, but corps-level assets which allowed it to in some respects operate independently. Corps still combined to fight their battles, but in principle a corps could fight more or less on its own.


There were two independent factors that combined to create the Napoleonic corps, in terms of size. These are the "span of command" and logistics.


The first, span of command, relates to the principle that a given commander can only manage a limited number of independent subordinate units - a rule of thumb is between two and four, it can be higher but you can't rely on that for doctrinal purposes.

The largest unit that can operate according to a typical regimental-level drill book is a tactical battalion, usually between about 200 and 800 men. A regiment with a higher strength on the field than this would be split into multiple tactical battalions, regiments with a lower strength would be consolidated into one tactical battalion.

French tactics in the Napoleonic Wars of course would involve larger formations operating together, but these were still built off battalion drill.

A brigade is (meant to be) a combination of multiple tactical battalions. Consequently an ideal brigade would be between two and four such units, and would have - at full strength - an on-the-field strength somewhere around 2,000 to 3,200 men.

A division is a combination of multiple (again, 2-4) brigades, in the same way, and also has artillery attached if the constituent units don't. This means a division would be hovering somewhere between 5,000 and 10,000 men on the field. (Strength PFD or AP would be higher.)

And a corps is a combination of multiple (2-4) divisions. It would have cavalry if the constituent units didn't.



The second, logistics, is actually related to why a corps can operate independently. Very roughly, a corps corresponds to a formation that can fit down a single road in a single day - which means that, if it runs into a much larger enemy army coming down a single road, the corps can hold out by itself for a day (at least, archetypically) because the enemy can only deploy a "corps" worth of units in a day.


This combination of factors is how you get the size a corps is "supposed" to be as somewhere between 20,000 and 40,000 men, with artillery in proportion and at least some cavalry assigned specifically to the corps.




ED: interestingly enough, the British system in the 1860s was based on constituent units only. They used a structure that was:

Each brigade was triangular (3 battalions)
Each division was binary (2 brigades) and also had 2 artillery batteries (of 6 guns each)
Each cops was binary (2 divisions) and also had a cavalry brigade (with 3 regiments and 2 horse batteries) and 2 batteries at the corps level.

Resulting in a corps strength of 12 battalions (though British battalions had a robust replacement system and the British logistics system didn't draw on line strength, so 12 battalions generally did mean about 10,000 effectives), but also with about 1,400 cavalry sabres and over 40 artillery pieces. While small, these corps were perfectly capable of operating independently and had serious punch at range.

 

[Josh The Lighthouse Guy]

Even within an army the size of corps could be very uneven. The AOTP lost something like 30 regiments to expiring enlistments between Chancellorsville and Gettysburg. I think some entire brigades ceased to exist.

For mostly political reasons, the Union often raised new regiments instead of replenishing existing units. Confederates were better about replenishment.

Neither side was good about rotating units. Confederate infrastructure couldn't support it, but the Union had the railroads and shipping they probably could have (and I would argue should have) cycled regiments between active and less active areas than they did.

 

[Cavalier]

Interesting, to me anyway, was how small the Irish Brigade was at Gettysburg. Per Busey & Marin, 532 engaged in five regiments. 28th. Mass. 234 engaged, 63rd.N.Y. 75 engaged, 69th. N.Y. 75 engaged, 88th. N.Y. 90 engaged and finally 116th. Pa. 66 engaged. I believe at this time the three original New York regiments were consolidated into a single battalion.

John

 

[Saphroneth]

Interesting, to me anyway, was how small the Irish Brigade was at Gettysburg. Per Busey & Marin, 532 engaged in five regiments. 28th. Mass. 234 engaged, 63rd.N.Y. 75 engaged, 69th. N.Y. 75 engaged, 88th. N.Y. 90 engaged and finally 116th. Pa. 66 engaged. I believe at this time the three original New York regiments were consolidated into a single battalion.

John

At that size, they'd fight as a single tactical battalion anyway. The drill book simply doesn't work when you have, say, 9 men per company.

 

[Rhea Cole]

The reason that regiments soldiered on with fewer & fewer men was that there was no central replacement depot to draw from. Regiments were state, essentially today's National Guard, units.

Officers joining the Nebraska Air National Guard, for example, received elaborate commission certificates exactly the same as Civil War era soldiers did.

In a tradition that dates to medieval times, regiments were raised locally. A county in Iowa would be given a quota of men to report. Proudly, there were no draftees to fill out the Hawkeye quotas. Rather than pooling all the men of a certain date, they would be formed into new regiments.

At the same time, serving regiments had to recruit their own replacements. Needless to say, hundreds of regiments attempting to recruit at the same time was wasteful & often unproductive. In that case the convoluted, often tragicomic draftee sagas come into the narrative.

On the CSA side of the line, in the Army of Tennessee, regiments from Missouri, Tennessee, Arkansas, Kentucky & Texas had no access to their home counties or regions. For that reason, those regiments were reduced to little more than companies or squads.

In the Army of Northern Virginia, the aggressive tactics that resulted in 40 to 80 percent casualties reduced some regiments to sergeant's commands. RE: 3rd GA & 23 NC at Gettysburg.

 

[huskerblitz]

I wouldn't go quite that far. While they were resistant to the practice, it did happen just not on the level as seen in the Confederate armies.

Ryan

This is true. The 32nd Kentucky was consolidated with the 22nd Kentucky in 1862, not too long after Perryville.

 

[Cavalier]

@Saphroneth I had the impression they operated as two battalions, one of 300 men and the other of 240. I have no idea how this would have worked while maneuvering on the battlefield however.

@Rhea Cole I believe that the reason the Brigade's previous commander, General Meagher, resigned was because he requested permission to return to New York and recruit more troops for the original three N.Y. regiments but was refused.

John

 

[Saphroneth]

@Saphroneth I had the impression they operated as two battalions, one of 300 men and the other of 240. I have no idea how this would have worked while maneuvering on the battlefield however.

Both of those are small, but it is doable. The general rule of thumb AIUI is that you fall off the bottom of the drill book under about 200.