So it's worked for me because it really fucks up the US AI. They can't figure out where to put their armies. As long as the British fleet will help you enough that the U.S. can't send ships to the Dixie sea node, you can basically naval invade Alabama for free. Just pay attention to the Naval Invasion indicator and when it shows that the U.S. has an army stationed in that HQ, cancel the invasion. If the war is close enough on the other fronts, i.e. you have the British and your main army occupying them in the midwest, the AI might send an army to station it to fight a naval invasion, but if you cancel the invasion they'll usually send the army back to the front. Once there's no armies in that HQ, there's no resistance for the invasion, as long as you don't have to fight their ships.

I kinda fucked up in the war strategy this run by taking New York, I thought it'd be good to build up and steal their GDP but it's just fucking radical city, nothing I build will raise their SOL, and it opens up two additional fronts in the north which the U.S. can easily throw armies at but which as Mexico, even with allies, I can't really do that.

p.s. Unless your armies can beat them 1v1 it's important to do hit and run shit, once the U.S. sends an army to stop your invasion force after it's taken a state, start another naval invasion in the same HQ as long as there's no ships or armies stationed in that HQ. Like take Alabama, and the day before they deploy to the front, naval invade florida to expand the front without having to fight them