Honestly, I think even the hoodiest of hood rats would make a good showing of it. In a theoretical situation where the U.N. helps, what troops would be sent? The Blue Helmets? An untested, inexperienced military unit with a divided command structure that gets the dregs of modern military equipment?
Well, those guys are going to find the average gang banger in Chicago has a higher body count than them and more experience in a gunfight.
Alright, those guys would be dead in a week if heavy fighting ever occurred. So who else do we got? Basically, the Military Police of various nations. No, not the full on military. No western country would want the horrid press that comes with sending in actual ground troops to invade the U.S., and countries like China would just laugh and do nothing because it serves their interests best. So see above for descriptions of those forces.
This is why it would never happen. It'd be a meat grinder, with no pay off.