I don't think, barring clear indications of imminent attack by DPRK anyway, that we would ever start a war with DPRK. The risks are huge for the US and much larger for ROK and Japan. Who is going to call removing the government of DPRK a victory if one or more major civilian centers in ROK or Japan is nuked? (I know, some here will, but the real consequences of that for geopolitics and the world economy could be huge)
With the way the North Koreans are suffering these days, you would think an assasination attempt would be tried.