The US will do the same thing it did when one of its other allied countries, Israel, got the shit rocketed out of it: nothing. This is due to present overextension and the fact that China might involve itself. South Korea will do nothing substantial, again, in fear of provoking a Chinese response...that is, unless they've grown some balls in the past sixty years. I wouldn't be shocked if they had access to atomic armaments, but that'd be bad for the US as the rest of the world would look down their long noses at us, presuming that we gave it to them. Then there'd be hell to pay.
China however will do nothing except be the 800lb gorilla in the room, thereby keeping the US out of it. Their intervention is ultimately unnecessary, as they will deal with NK if Kim ever threatens the Chi-Coms, which he won't if he wants to keep living.
By all rights, South Korea should be made into an island tonight. But it won't happen. Kim Jong Il and his darling son will also remain in powet, if for no other reason than half the North Korean people would probably commit suicide if they were removed.
