A couple of points.
NK is NOT a the old USSR. Despite its Stalinist ideology, NK has no interest whatsoever in being an expansionist power beyond its stated desire to "Unify" the Korean peninsular. NK actually wants as little as possible to do with the outside world (They have an ideology of Juche, basically extreme Autarky. The idea of NK wanting to occupy lands other than SK is basically risable)
As such, NK is no "Offensive" threat to anybody else whatsoever other than the South. NK will NOT launch any first strike long range nuclear attacks against the USA or anybody else for that matter, why would they?? Doing so would serve no purpose and bring Hell-Fire raining down on their heads.
If NK did want to make some sort of pre-emptive attack on the rest of the world, I actually see a rather more passive (and yet considerably more damaging) way of using their missile tech (That would actually be very difficult to respond to)
If I was NK and really wanted to **** things up for the rest of the world (and in particular the USA) I would use My missiles to launch a couple of hundred Kg of builders sand into "Wrong Way" geo (and other) orbits.
NK wouldn't care one jot if there was no more Sky TV, Spy-sats, Com-sats or GPS ever. The rest of the world would have course be hopping mad over this, but how, realistically, could we respond?
(Pretty much any aggressive response that would have any lasting effect would result in the deaths of, literally Millions of innocent people, Would a society that prosecutes a soldier who executes a mortally wounded prisoner for Murder have the will for this sort of warfare today?? Hell no!)
As far as offensive use of Nuclear weapons is concerned I can easily see NK using "Neutron Bombs" as part of a future assault against SK. All the NK Nuclear tests thus far seem to have generated yields that are right on the button for Enhanced radiation devices (7-10Kt, anything larger than that and the blast effects dominate) especially since they have claimed to have produced a fusion device of around this yield. I certainly think that this is likley to be the main thrust of their nuclear weapon development program.
Although, NK makes much propaganda of their missile and long range Nuclear capabilities (They have missile subs too). The only use of this capability I could envision is as a retaliatory strike If the US invaded or struck the North to such an extent that they felt they were losing. NK's Nuclear tipped ICBM's really are only for deterrent/defensive purposes.
I really DO NOT want the US to take any military action against NK. For now containment is just fine. I do not regard them as any particular global threat unless attacked.
Even if NK did invade SK, I would rather we all kept out of it. This is not the 1960's, there is no "Domino Theory" doctrine going on here. I would be sympathetic towards the suffering of the SK people certainly should such an invasion occur. But not to the extent that I would be willing to risk a global nuclear exchange from defending them.
No, No Way.