You could easily have the pretext of remaining NATO forces in Europe being pulled back to regroup on US soil. It would add some interesting backstory potential, with many soldiers having to leave their home and families behind for an indefinite amount of time, trying to live among masses of international forces in a familiar, yet foreign land.
This is the most non-sensical thing I could imagine though
Why would European nations leave Europe to fight on an entirely separate continent when the battle is in Europe (which is where their own countries are) and the U.S maintains a large military force in-country, which is much larger than their expeditionary force.
The U.S is entirely capable of defending both itself and being present on two fronts without needing to wring the brain and suspend disbelief to a point of seeing European nations pulling back to the U.S
If anything, allied nations would break apart and focus on defending their own respective nations in the event of a U.S pull-out and a "doomsday scenario"
If you start a server with the scenario that European NATO nations have pulled back to the U.S to defend the land, then you start off the server with NATO already having lost the war