But...But...North Korea managed to invade the US in Homefront!!
Yeah, I winced too.
Invading the US or Australia wouldn't be too tricky given the right amount of commitment to the operation and a liberal dosing of tactical and strategic nuclear weapons however occupying it...pfft, forget it.
Ain't gonna happen. You might get some guys flocking to your side depending on the propaganda, but you'd have to committ genocide on an epic scale to pacify the US.