67th Tigers
Banned
As to the basis of the Union being able to fight off the British and French, as well as the South in 1864. Certainly it is arguable, very arguable, from either position. The South certainly didn't have the manpower to conquer any state at any point in the previous years when things were going relatively well. The British might be able to concentrate the resources for temporary occupation of areas, but the French sure can't. They have too much at stake on the European Continent to deploy much more than they did historically in Mexico, which note is not remotely conquered, just partially occupied.
So really, the only nation with the wherewithall to seriously threaten the Union is Britain. Lee, Longstreet, and the other Southern Armies can tie down large numbers of Union troops, but there simply are not the necessary manpower reserves after losses to attrition and combat in any extended campaign for them to do much. If the Union uses economy of force, which it could very well do in this situation, it could mass sufficient troops to fight the more dangerous enemy.. in this case the English. All the Union has to do is wait for developments in Europe or elsewhere to draw off the English and French (which are alluded to in the books) and then the Confederacy will lose. It will add years, but probably not more than one or two.
Keep in mind that Grant complained in 1864 bitterly about tens of thousands of Union troops that were not doing anything useful. There are reserves available, and Tsouris has also given the Union a patriotic second wind in the books. A not unreasonable one really, considering the British have launched an ill advised, marginally justified war to begin with.
The French Army of 1st January 1863 has 404,400 effectives with the colours (meaning enlisted troops, excluding officers and employees). It was a relatively small army compared to say, 1860 (559,902 in that category) or 1856 (526,056), but the French Army strength varied depending upon the demands placed on it. Their conscription system only took a small proportion of the fit men in each class. For example, in 1863 the breakdown of the enlisted men is:
65,911 Volunteers
111,266 Re-enlistees
171,412 Conscripts
1,141 Substitutes
36,505 Replacements
18,435 Musicians
In the previous seven years (the length of a term) 740,000 had been enlisted (100,000 in 6 of the years, 140,000 in 1859). Of these over 400,000 were released back to civilian life as reservists (commuted), but were still liable to recall. The class of 1864 could be drafted at 140,000. Whatever the French Army needed could be pulled from those commuted (as happened in 1854 and 1859 - the latter seeing an immediate and sudden expansion of ca. 200,000 men).
Should the French decide to send 250,000 men to America, they could do it and maintain their strength in Europe with a reasonable reserve.
I'm not sure where these Union reserves are meant to be. Nor where they aren't matched by a Confederate force. At the time of Gettysburg the approximate strengths were:
Army of the Tennessee 75,000 and 16th Corps(+) 25,000 (guading supply lines) vs Army of Mississippi 56,000 and the Army of the West 32,000 and 5,000 cavalry
Army of the Gulf 20,000, besieging Port Hudson 8,000
Army of the Cumberland 65,000 vs Army of Tennessee 45,000
Army of the Ohio 15,000 vs 15,000 detached from the Army of the Tennessee
Department of Western Virginia 10,000 vs Army of Western Virginia 8,000
Army of the Potomac 90,000 vs Army of Northern Virginia 75,000
Dept of Virginia 15,000 facing Richmond Defences 12,000
Dept of North Carolina 15,000 facing Dept of South VA and NC 12,000
Dept of SC 17,500 facing Dept of SC, GA and FA 11,000 at Charleston
Plus 30,000 in the Washington defences, plus the trans-Mississippi depts etc. (and the force at Mobile)