Then what the hell do you call "arming Japan with nukes"?
A nuclear Japan isn't any more likely to launch a preemptive attack on Pyongyang than a nuclear Washington is. In fact, it's probably less likely to.
Of course. NK knows that too. They also know that if they attack first, they will be instantly marginalized from the rest of the world, who will rush to our side, because we're the only nation on the face of the Earth willing and able to deal with the problem.
There's all the difference in the world between having an American and a Japanese finger on the button. The US long ago said that any attack on Tokyo would merit a nuclear response from the United States. That should be restated and made explicit. But I'm still not sure how allowing Japan to develop its own nuclear weapons makes things any better, and I do see how it makes things a lot worse.