A United States. You wouldn’t say “an uniform”. You would say “an underling”. First one has a y sound to start, second has a u sound to start. The usage of a/an depends on the next sound, not the next letter.
I mean, it absolutely could. California alone supplies the planet with a massive percentage of food. If the this image was accurate, we'd have a surplus of food. We might just have to tolerate losing a lot of stuff we can't (?) grow, like bananas. Or they'd be remarkably expensive because only Hawaii or Florida or somewhere that could grow them.
Power? Again, we have collosal amounts of energy or energy potential. From green energy to fossil fuels and natural gas. Without selling it to our allies, we'd have a surplus of energy.
This is literally why America is the superpower. But a lot of countries could likely do this. Russia could for sure too.
Economically, however, I can't even speculate.
Fun thought experiment, worthy of a Kutzgesagt video!
Edit: I thought you said "couldn't" but I already posted so 乁༼☯‿☯✿༽ㄏ