I’m not sure if its been mentioned yet, but I don’t think the US has to worry about an invasion. First of all, the chances of the number of enemy troops that would be needed to conquer the US to actually make it into the country are very slim. The various branches of military and the far more advanced and more powerful weapons would make that nearly impossible.
Now, even if a couple hundred thousand troops manage to make it in. They wouldn’t be able to last very long with the troops still in the US, the police, gangs, hunters, military veterens, and every Tom, Dick and Harry carrying guns. Especially, in the South where people are still driving around with shotguns hanging on gun racks in their pickup.
Anyway, I think the only way any country would be able to conquer the US, would require WMD, and lots of them.
Other than that, I would imagine Japan would help us contain North Korea.