Chinese Nuclear Retaliation which would likely wipe north korea off the face of the earth, Not only that but heavy devastation across the nuked land in china and north koreas ascensionship to nuclear crater. even if this doesnt happen north korea would collapse with china due to china supplying north korea with money, food, and most importantly, pretty much all of its resources. this action just dooms your own regime to chaos. Also china would never agree to loosing everything.
Thanks to @Dalegrad for the RP Project and @Jacqueline-le-teacup for the KPA project.