So I tried to address a similar problem with the Monroe Doctrine. My group and I still have not gotten Monroe reaction by US down to 100% because the rule designer clearly didn’t think or test this shit out enough. (not shocked based on responses).
Non allies attack Americas: I’ve been told that the US doesn’t actually DoW by definitions of the words but take military action while at peace, think of it as a proxy war but both sides are directly involve like the Suez Crisis in real life. US may go down and engage and remove any hostile nations in the Americas and then they end up annexing the territories they took since they are active by rules and thus US was able to gain income while still being at peace.
France or Commonwealth (or any other official Allies): I put in the house rule that they cannot attack the Americas because the game doesn’t allow nations inside Alliances to fight each other so unless Monroe is the only situation where US will fight French and Commonwealth troops, I tell my group they simply do not wish to piss off the US and thus cannot attack the Americas.