There hasn't been a nuclear strike since '45 because the USA and the USSR/RF have been operating per MAD. MAD doesn't apply to a rogue, unpredictable state like NK.
There's a problem with this logic though that there isn't any objective definition of a "rogue, unpredictable state." When the Soviet Union was involved in the Cuban missile crisis, the outcome for that situation was pretty much by all accounts far from predictable. I'm not saying that it's impossible that North Korea might do something, but it's not like that's something new in world history. Plus, there are some pretty good arguments analyzing North Korea's history to suggest that they have actually acted rationally from their point of view.
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