The three large field commands of the US army of OTL, Jan 1862 (PFD strengths as of Dec 31 1861)
Department of the Potomac (183,000)
- HQ
- Banks (already in New England)
- Blenker
- Casey
- Dix (really the Baltimore and Annapolis garrison)
- Franklin
- Heintzelman
- Hooker
- Keyes
- Lander
- McCall
- McDowell
- Porter
- Smith
- Stone
- Sumner
+sundries (e.g. cavalry)
Each division very roughly 11,000 strong, except for Lander which is larger
Department of the Missouri (91,000, is AP not PFD)
I can't find the division list but there'd be about seven-eight divisions of the same size.
Department of the Ohio (72,000)
There'd be about six divisions of the same size.
In March the above were combined under Halleck's overall command, and were:
-Grant
- McClernand
- Charles Smith
- Lew Wallace
- Hurlbut
- WT Sherman
- McKean/Schofield
-Buell
- George Thomas
- McCook
- Mitchel
- Nelson
- Crittenden
- TJ Wood
- George Morgan
-Pope
- Paine
- Stanley
- Schuyler
At sixteen total, these are probably about 10,000 each. If I've missed units, I'm not missing men as I'm dividing the men known to be there by the number of divisions I've identified.
There are also three expeditionary forces of roughly divisional strength - one (Sherman) already at Port Royal, one (Burnside) at Fort Monroe ready to go to North Carolina and one (Butler) still in New England.
TTL's basic appreciation was that I stripped down the Dept. of the Missouri heavily to face Canada (pulling out about 60,000 from it) and cut another 60,000 from the Dept. of the Potomac. Of these 90,000 total went to Canada and 30,000 to the coasts.
I think it's possible that I would alter this - if nothing else I've discovered there were probably more Confederates facing the Union in Missouri - but the number facing Canada and the number on the coast are both minimal, so those troops have to come from somewhere.
Starting with the expeditionary forces, I had both Burnside and Sherman be caught out of place.
As far as I can tell the Union proceeded with the preparations for Burnside's expedition even while Trent was an uncertain thing, but I might make it so Burnside's force (or part of it) is pulled back without the release of tensions. His and Butler's forces are the obvious ones to go to trouble spots straight off (specifically Butler's division to Maine and Burnside's to Rouses Point?) while troops start trickling up to form an army on the Canadian border. On the other hand, I also had Sherman's force partly get back, and that IMO is probably quite unlikely - so it's partly a wash.
The minimal level of coastal defence is, effectively, a division each at NY, New England and Philadelphia on top of what's already there (the troops present being able to handle manning the forts but not a lot more). This is incapable of standing off a serious British landing except by being right there in brigade strength (with reinforcements to rush to the scene), though, so more is probably quite likely. NY, for example, might need two divisions (one for the city proper, one for Long Island).
Coastal positions can support armies of essentially unlimited supplies (or they can south of Maine, at any rate, Maine is a bit more vulnerable because there's only one rail line and it's near the coast in several places).
I estimate the requirements sent would be Philadelphia (1), NY (1), Boston (1), Portsmouth (1) and Portland (1), with in practice a lot of the disposable brigades shifting north once the Portland operation takes place.
Total five divisions required.
Facing Canada, the main point of effort is the line of Lake Champlain. That's the best avenue to push up into Canada and hit Montreal.
I'd say the limit on what you can support there is on the order of 40K or 50K, because of the quality of the rail lines. Say four or five divisions.
Buffalo and Detroit can both support quite large forces; of the two Buffalo would be "offensive" to try to cut the Welland and Detroit would be "defensive". My estimate is that these would be about 20K each (2 divs).
And a small force on the line of the St Lawrence (1 div) to try and interdict it.
Total 9-10 divisions required.
Grand total 14-15 divisions.
So here's how I'd move them around, I think:
-Pope's army is kept to two divisions and is essentially in a defensive role. The battle of Island Number Ten is not organized. (Paine's division goes to Detroit)
The remainder of Pope's army is there to prevent the Confederacy getting back into Mississippi, but is likely to be raided again later if reinforcements are needed.
-Grant's army is cut in half, to three divisions (losing Charles Smith, Hurlbut and Lew Wallace?). These go to Detroit (Hurlbut) and Buffalo (Charles Smith, Lew Wallace).
Grant's army might be materially capable of Henry and Donelson, but I doubt it would be allowed to as it would uncover a huge fraction of the frontier (as no more troops are available to transfer in). A defensive posture is more likely.
- Buell's army of the Ohio gives up three divisions, because of the need to keep at least one strong force in the west in case of things getting nasty. Nelson (to the St Lawrence), TJ Wood (to NY) and George Morgan (to Philadelphia).
Buell's force is intended to move to provide reinforcements where they might be needed west of the mountain spine.
- Rosecrans in the Shenandoah area gets his men back (essentially Lander's division) as the AotP cannot afford having their flank turned now
- The Army of the Potomac needs to give up about seven divisions to make up the numbers, which will reduce it considerably. It will get most of the recruits in the pipeline, but in return will see the transfer of Banks (to Boston), Sumner, Blenker, Stone (to Plattsburg area), Porter (to Long Island Sound area), Casey (to Portsmouth) and Heintzelman (to Portland)
The AotP has to stand on the defensive, and though it swells back towards the old size a
lot of them are new recruits with poor weapons.
What that leaves the US Army at is:
Facing CSA (McClellan in command of the east, Halleck the west)
Army of the Mississippi (Pope) (under Halleck)
2 divs (Stanley, Schuyler)
Army of Western Tennessee (Grant) (under Halleck)
3 divs (McClernand, McKean, WT Sherman)
Army of the Ohio (Buell) (under Halleck)
4 divs (George Thomas, McCook, Mitchell, Crittenden)
Army of Western Virginia (Rosecrans) (under McClellan)
3 divs (Rosecrans, Cox, Lander) - Rosecrans in the Cheat Mountain valley, Cox in the Kanawha valley, Lander at Harpers Ferry
Army of the Potomac (McClellan)
6 divs (Franklin, Hooker, Keyes, McCall, McDowell, Smith) plus new recruits
Coastal defences (no overall commander, though much under Dept. of New England )
Baltimore (under McClellan)
1 division (Dix)
Philadelphia
1 division (George Morgan)
NY (under NY Gov and Major General Morgan)
1 division (TJ Wood)
Long Island Sound (dept of NE)
1 division (Porter)
Boston (dept of NE)
1 division (Banks)
Portsmouth (dept of NE)
1 division (Casey)
Portland (dept of NE)
1 division (Heintzelman)
Facing Canada (under Fremont)
Maine (dept of NE)
1 division (Butler, also dept commander)
Plattsburg area (Fremont)
4 divisions (Burnside, Sumner, Blenker, Stone)
St. Lawrence (under Fremont)
1 division (Nelson)
Buffalo (dept. of NY) (complex command structure) (local commander Harney)
2 divisions (Charles Smith, Lew Wallace)
Detroit (local commander Robert Anderson)
2 divisions (Hurlbut, Paine)
The shift to this configuration would not be instant and the logistics would probably take some weeks to sort out.
The two local commanders assigned above are unassigned BG of Regulars, but were both humiliated early in the war with the CSA. I assume they're brought back in because of the war with Britain and assigned to anti-Canada commands.
I admit the above is provisional, and that it doesn't explicitly list militia or some below-division level commands - but they're likely to be taken up with tasks like bridge guard or garrisoning forts.
It is not entirely consistent with how the TL is described - that's because this is me sitting down and working it out now. Do let me know if you detect a mistake! (for example, if there's somewhere I've overly injured the US.)