I doubt Puerto Rico will become a state right now, because as a territory we get to tax them without representing them in congress. I'm unsure why they put up with that; maybe they just want protection. Say what you will about the rest of the world, but the U.S could likely curbstomp any other country on this side of the world, although maybe not a full alliance. I think Brazil is the strongest State (Capital S) in the hemisphere, although I've heard Canada is surprisingly badass for having a population measured on one hand.
Still, the only countries that would have the power to do some damage on their own are...
China (The second-biggest power in the world, could possibly be THE biggest but I think the economic manipulations they're doing would hurt them in a war)
Russia (?)(I'm not sure how well they stood up after the USSR fell. They're still a big player, I know that)
Germany (Proven time and again they're a national version of David, but without the victory record. They could hurt us, I doubt they'd win)
U.K (Would very, very quickly become a defensive war on their side but the U.K is made of friggin' water; no compression, no conquering)
Japan (If they geared up for war, they'd have an economic advantage over even the U.S, and they're tough little buggers too. I still think the U.S would win, but the casualty count on both sides would be huge.)
France (?)(If you count an unending resistance as continuation of a war, then France counts. Even we couldn't hold them down, even though we could nick their lunch money easy)
I can't think of any others, but I'm sure there are some out there. Any suggestions?