Still working within the black magic world where NK is adjacent to ISIS-land …
I’m explicitly assuming what it seems most folks have been implicitly assuming: This war takes place in a vacuum where all the outside powers just pull up a chair to watch, in fact they pull their current real-world forces out entirely. And in this vacuum jihadi recruits from around the world in general and the Mideast in particular aren’t permitted to join the fray. If one relaxes either of these conditions I don’t know where else one could plausibly draw the boundaries and still have an interesting hypothetical.
With those preliminaries out of the way …
A good look at a globe shows the gross territory under ISIS influence now is about 5x the land area of NK. Over history at large, we have few examples of even heavily militarized societies successfully conquering that much additional territory.
OTOH, ISIS’ territory consists of a lot of truly uninhabited (not just sparsely inhabited) wasteland plus some towns, cities and agricultural areas tied together by networks of roads and villages along those roads. Which is why a small force like ISIS can tint so much of a map: they aren’t actually present in most of the tinted area, but neither is anyone else.
*If *NK limits its action to running up the same road network and overrunning the same populated areas they’re looking at controlling only roughly 1x their home territory. Which is historically plausible in and of itself.
But all that trackless waste is also a place for ISIS to hide and raid from. ISIS would be hard pressed to sustain themselves for long out there, but so would any NK pursuers. I see ISIS having some of the same kind of success they had against the local legit Arab armies: Harassment actions and localized massing to force isolated outposts to retract.
Said another way, a roads-only approach will leave NK forces with a huge surface area to protect. Competent armies generally want a compact area of control with few salients and no outposts. US forces can get away with a more outpost-oriented posture when we have overwhelming logistical and air cover to support and protect the perimeter of each outpost.
Both ISIS & NK forces are very weak logistically compared to typical First World militaries. And the third enemy of both is the shear harshness of the climate and terrain.
My revised bottom line:
Upon further consideration, I see NK can defeat ISIS wherever ISIS chooses to stand and fight. Since ISIS has already demonstrated a good grasp of asymmetrical warfare against several differing types of forces, we can assume they won’t do that unless cornered.
Absent set-piece battles, NK will sojourn into the desert & be swallowed or bogged by a mixture of terrain, ISIS harassment, and logistical shortcomings. Much like the periphery of the North Africa campaign in WWII; both sides fought the land almost as much as each other.
Kim’s Glorious play for People’s Lebensraum stalls pretty quickly then stalemate and attrition guerilla warfare set in. Kim eventually tires of (or can no longer afford) the meat grinder & NK goes home. Not much is left of ISIS by that time.