From Steven Z. Starr's The Union Army In The Civil War: From Fort Sumter To Gettysburg, 1861-63:
The thing that I'm not seeing from this is cases of actual assignment of regiments or sub-regimental organizations to infantry brigades - we're told that it happens, but not given examples (unless of course I've missed something about what Starr has said).*
I am aware that what's going on here is that Starr is reading the fact that SO 90 doesn't assign a brigadier to the cavalry attached to the 2nd, 3rd or 4th Corps as evidence that McClellan didn't intend to brigade them at all, but the fact that there's a space for a BG to be assigned to command Sumner's cavalry seems to indicate that he was thinking in terms of appointing BGs.
Each of the cavalry organizations attached to a corps has a colonel for each regiment, and there's a senior colonel; based on the way McClellan did things before the formation of corps (with multiple DCs reporting to a single DC) it seems at least plausible that McClellan was intending to not formally elevate someone to brigadier general or give them formal corps command until he knew which of his untried cavalry commanders was actually capable. (Hatch is already a BG(V) so he gets a brigade command.) Yes, McClellan appoints people to command brigades and divisions despite not having the experienced men to do so, but the largest formation of infantry you're going to see is the corps; the largest formation of cavalry you're likely to see is a brigade or possibly division, so it's those men you need to be the most careful about picking.
For Starr to describe Hatch's brigade as "simply a training organization" seems to me to be incorrect, or at least to not be something related to McClellan - he explicitly specifies Hatch as a brigade commander in SO 90, albeit of a different brigade (1st NY, 2nd NY) which he plans to take to the Peninsula (but doesn't get to because they're assigned to 1st Corps and stripped away from him when that force is), and the 1st VT and 5th NY are both assigned to Banks (i.e. to the same subsidiary command).
It's also the case that, whether or not Stoneman's official role was meant to be administrative-only, as a matter of fact he commands the cavalry in the field at Williamsburg and subsequently operates the Advance Guard on the move up the Peninsula. So it seems likely that his role was meant to be cavalry field commander when that was needed.
In SO 90 McClellan has given each corps 2-3 regiments and grouped the rest as a consolidated two-brigade formation. If McClellan had consolidated
all the cavalry at that time (March 1862) he'd have been literally the first Union commander to do so, because even Pope hadn't done it yet; given that at this time Stuart wasn't commanding a consolidated cavalry formation he may well have been the first Civil War commander to do it full stop.
I think that criticizing McClellan for not consolidating all his cavalry into one formation is a bit unfair, on that basis; if it were already standard practice then maybe, but at this time (Island Number Ten ORBAT) Pope's cavalry consists of:
- One regiment attached to 3rd Division
- Two companies attached to 4th Division
- Two regiments formed into a Cavalry Division
- One cavalry regiment, plus one organization of four companies and one of three companies, all unattached.
So it's clearly not yet standard practice, while Napoleon's infantry corps each contained multiple cavalry regiments as standard (on top of cavalry corps as consolidated formations of cavalry alone).
Of course, on the Peninsula McClellan simply doesn't have enough cavalry to form a brigade per corps and have some left over for consolidated action, so those corps brigades don't get any commanders assigned because they don't exist.
The regiments you are talking about are also often far from full strength. Starr sees them as the equivalent of 9 and a half regiments when he adds things up.
Is he adding up in PFD or in terms of the number of companies? Most Civil War units aren't at full establishment strength for most of the war, but I'm pretty sure the company totals in many of the regiments are 10 or 12 companies in general; if of course Starr is counting
while on the Peninsula then of course it's going to be a smaller total, many of the 14 regiments never made it.
Of course, it's possible I've misunderstood here. But the fundamental question which I'd like to see answered - which is "how was McClellan's cavalry actually assigned and what was it actually doing on the Peninsula, in numerical terms" - doesn't seem to be covered.
* note that if a brigade of infantry is doing something and has a squadron of cavalry seconded to it for support on a temporary basis, that is actually appropriate because it avoids divided command in the small unit action and because committing a whole regiment to a small unit action is overkill. If a brigade of infantry has a squadron of cavalry attacked on a permanent basis,
that is a problem.