What impact will the USA have, if any?

Keep it simple stupid.

If the game is rock-paper scissors, you don’t make a new rule about wind disposition and rolling a d20 to resolve critical hits from the rock. It just looks ridiculous IMO.

The rule addition has to have the “look” like it belongs to the game and does not effect one player only or it may upset balance.

Its’ like wearing an Armani suit with a yellow 5 gallon foam cowboy hat. You just don’t do it.

How about for US entry, a track from 0 to 6 where the US starts at 0 and for every IPC they lose to unrestricted sub warfare, they go up 1. After they go above 0, they roll a die each turn and enter the war if they roll at or below the level they are on. For example, of they have lost 1 IPC, they enter when they roll a 1, if they’ve lost 2 they enter at a 1 or 2, and so on with automatic entry on turn 6. Any turn the US is not at war, they gain 1/2 income with a 20 IPC bonus upon entry.