Someone needs to say this. And I'll probably get hammered for doing it. But Nuclear Weapons are like guns - they are the greatest equalizer.
It doesn't matter if you're a big nation with a large army, or small one that can barely defend itself. When you have an atom bomb that you are willing to use, it changes everything.
You become part of a whole new class of nation. Also there's no way for a large military power like the US to invade you without suffering unbelievable looses.
That's North Korea's true motivation. And that's the motivation of most other rogue states.
I'm expecting Iran to come next.
The question is how should South Korea and Japan respond to this? Whether you like it or not, I'm willing to bet that before long Japan and South Korea will have nuclear weapons too.