Kim Jong-un has threatened nuclear strikes on South Korea and the US. Clearly he does not fear the prospect of a US retaliatory strike turning North Korea into an archipelago. What he fears is peace.
The world’s only hereditary Communist monarchy places extraordinary demands on its people, subordinating their well-being entirely to the needs of the state. If peace were to break out on the Korean peninsula, and people and information were to move more or less freely between the two Koreas, the North Korean regime would lose legitimacy, and so would its nuclear weapons program.
The alternative to peace on the Korean peninsula may be regional nuclear proliferation. If the threats from North Korea exceed the credibility of US security guarantees, both South Korea and Japan could develop their own nuclear deterrent forces. China and Russia know this, and do their best to let the North Korean pot boil, but to keep it from boiling over.
But North Korea is trying to develop ICBMs that can deliver a few nuclear weapons onto US territory. At that point, the American people may prefer that Japan and South Korea take care of their own problems with North Korea. Perhaps China and Russia might prefer peace between the Koreas to new nuclear powers on their doorsteps.
Of course, North Korea will not entertain the idea of peace with the South unless it has no other choice. Essentially, the US with the permission of China and Russia should play the bad cop, while South Korea plays the good cop, saving the North from US aggression by establishing warmer relations.