re: Hypothetically, what kind of money/force would it take to commandeer NE Mexico?

I think that's your answer. The problem wouldn't be taking it, as much as it would be stopping the cartel insurgency from retaliating. There would need to be the expulsion of anyone that didn't agree with the new program of frowning on the bribing of public officials, etc., otherwise it'd be dead in the water.

But someone like Sleepy Joe would just surrender it after we took it anyway.