By Colin Salt
When making alternate armies that are far larger than their real life counterparts, it's important to note the important bottleneck that can't be easily solved by just having large amounts of equipment or people. Even having outside aid doesn't counter the need to have more than just the "ingredients".
Just as eggs, butter and flour mix don’t equal a pancake on their own, having three hundred tanks and two hundred APCs does not equal an armored division. By American standards, even in World War II, it took a year and a half to turn a scratch-built division from “exists on paper” to “ready to deploy”. Postwar, two years was an optimistic hope.
The central core of the division is a “cadre”. The officers (how many are commissioned vs. non depends on the circumstances) comprise the cadre of the division, which in a normal sized division is around one to two thousand people. Reserve forces deliberately keep their cadre at higher strength and readiness so they can be quickly built around during mobilization.
There’s also external powers not just supplying the equipment, training, and resources for their client/colonial army, but also supplying the central officer cadre as well. (This is why one snapshot analysis of Iranian casualties in the Syrian Civil War found no confirmed dead below the rank of sergeant.)
I also want to say that the cadre forces are the hardest to obtain (from just my amateur gut reaction) compared to either a large number of shorter-trained recruits or a few high-level commanders. The bottleneck for your revved-up 25 division army, especially an effective one, feels like the roughly 25,000 people in the cadre, compared to the 25 commanding generals or the 250,000 enlisted. And this is before political difficulties arise. Although I should note that this was less an issue for continental powers in the World Wars because of both having plenty of survivors from destroyed units as cadres and, to be frank, lower standards.
Cadres can come from:
Scratch-trained officers “jumping rank”.
The small existing military being streched out to become a cadre force (this happened in World War II-in 1939 Ike was a lieutenant colonel).
Retired personnel being brought back.
Existing irregulars being formalized. (This led to “Zouave” regiments in the American Civil War being formed around Napoleonic reenactors because they were the only ones with skill at musket drills).
External personnel being sent in to fill the cadre role.
Survivors from reduced/destroyed units.
Colin Salt reviews other genres at his blog: Fuldapocalypse Fiction and has written The Smithtown Unit and its sequel for Sea Lion Press