-They have a massive military - in sheer #s even with mediocre tech - and are a stones throw away from 2 significant allies in SK and Japan.

-They are a nuclear state well on their way to developing ICBMs. The fact that they keep expanding their nuclear capability is basically all that's kept them in power this long ... they're not stupid; they're going to keep forging ahead.

So it's a massive detente in that it's not really worth (in blood or treasure) taking them out because of the damage they could inflict in the course of our doing so. It's also unlikely we'd be able to get much support from allies, though SK and Japan may be roped in by virtue of being attacked themselves. But OTOH can we continue to let a rogue state stockpile weapons of mass destruction while developing the tools to deploy them anywhere on Earth? Where's the red line? What about the people that are tortured, go hungry, etc. under the regime? We just ignore them?

The curveball is NK's propensity to deal with other rogue states and terrorist groups. IS getting a nuclear warhead is a scenario too tragic to even begin to consider... We'd likely be looking at a major Western city uninhabitable for decades...