Japan will realize this on their own soon, especially if the american electorate hands the Dems power.
The very reason we trust Japan as an ally is the same reason they opt out of having nukes on their own accord.
They beleive in stability, and their developing a nuke would simply add to the instability. Of course, they live in a world where not enough countries feel the same.
Ultimately, I think if Japan was nuked by Korea, we would then nuke Korea. The Japanese and probably the Koreans also think this, in my opinion. Therefore, since they would never preemptively nuke another country, they can have their cake and eat it too, without seeming agressive on the world stage.
Meanwhile, they can build any number of "Defensive weapons" and I have no doubt they will.
France has a small nuclear arsenal. In a strategic nuclear exchange with any super power she would lose. However, Charles Degaule postulated, would the United States go to war and sacrifice New York for Paris if France was attacked? If I were the Japanese I would get very busy building a nuclear deterrent under their control.
I loved the Japanese response that if Japan wanted nuclear weapons they could develop them in just a couple of months.
No way I just say we station a Nuclear Sub near Japan on stand by.