Well, North Korea is part of the Axis of Evil (with Iran and Iraq) so you would think they would be near the top of the list of countries to invade. But really, who polices the US? What's going to happen when the US annihilates all of the countries that are developing nuclear weapons, leaving America as the only country with them?
We're all doomed.
I've got a question for you: isn't one nuclear superpower better than two (or three or four or five, etc)? Sure, they might have total dominance over the rest of the world, but at least they won't have any legitimate threat on which to actually use their nuclear capabilities.
I reckon that if it so happened that the U.S. were the only country in the world with nuclear capabilities, I wouldn't find that so bad. It's not that I don't like the fact that other countries have nuclear programs... what DOES worry me is that most of those countries are not Australian allies. At least the U.S. have strong ties to us in a lot of areas, so we're safe if they end being sole nuclear power.
Besides, even if it DOES happen, there will still be Russia, the UK, France, Germany, China etc to keep them in check (even without nuclear weapons, a coalition of the majority of the rest of the world is sure the convince the U.S. to keep their feet on the ground).
But having said that, I absolutely don't agree with the U.S.'s stance of pre-emptive strikes (akin to terrorism) against other countries who have not explicitly threatened anyone. The whole notion of "liberating" other countries and "helping them move towards the American ideal of how life should be" really shits me.
So I would disagree if the U.S. decided to invade North Korea (or Iran for that matter), but once it's all said and done I wouldn't be too miffed that the have one less nuclear-capable enemy.