I am heavily abusing naval invasion mechanics and how they absolutely brainfuck the AI. I still almost lost this last war (liberating New Africa and taking Pennsylvania) because Great Britain's idiot fleets were set to "defend naval invasions" and wouldn't help me
I just played a game as Mexico but failed, how do I deal with the UK allying with the USA?
You don't, if that happens you're fucked. You could ally with France and hope for the best, but that's probably not good enough to overcome a US/UK alliance. The way to beat the US as mexico is to get yourself a European ally, build up heavily and abuse the naval mechanics.
Ok so I managed to do it in 2 wars, first was Mex+Fra vs US+UK, I lost but only 2 states with 0 buildings, by the next war I also had a defensive pact with Russia and managed to win, In the next 2 wars I should be able to get all of USA.