USA Entry into the game (Decision in the West) is hardwired to the December II, '41 game turn. This makes things considerably for the whole ETO project. Soviet entry is variable (thanks to the Stalin bot), but the USA enters ETO via the Pacific Theater, so it is a fixed event. Here is a peek at what the USA brings to the game upon entry:
Right, Jay. USA Corps 25-36 were never mobilized. They were the USA's "100 Divisions Plan." Instead, America used more of its manpower expanding air and naval forces (the Arsenal of Democracy plan) rather than drafting a massive army. But we'll leave that the player's decision to make as to what mix of forces to build. There are only so many USA RPs, and you can't have everything.
Most of those CORPS will most likely not be built. If the U-boats are causing loses, you better be building Cargo Ships. New planes require 2 PPs and they might have an easier time attacking the Axis. The Allied player must build what they need because they can not build everything.