The biggest success of diplomacy is in avoiding war and so far it has worked in your hypothetical example.
I know you are talking about North Korea. That regime has nukes for the same reasons that everyone else does; national pride and d**k swinging. They also want to re-unite the Korean Peninsula under their regime, at least in theory. In practice that is not possible and there is no-way that can happen because they don't have the power and also presumably don't have a death wish.
China is the key here and they are NKs one ally. It has been argued that China let NK flex its muscles in order to act as mediator and score diplomatic points etc. NK is entirely economically dependent on the Chinese. Sanctions or even tariffs from Bejing would utterly cripple the NK economy and rest assured that China does not want to risk nuclear war in the region.
What can work in terms of progress (if disarming Pyongyang is what you want) then a strategy of building better relations with the Chinese is the way to go..
