> And if north korea was a chinese vassal, they wouldn't have nuclear weapons. It's funny how people want to ignore the obvious.
I agree with this view unfortunately. I find it a very harsh view. It's too harsh for my taste and I have the strongest wish that I didn't view things this way. With that said, the evidence is too strong in my opinion.
The evidence that supports it is that Ukraine was invaded by Russia, because they didn't have nuclear weapons. Ukraine used to have them and they were not invaded then. This means that nuclear weapons act as a deterrent. It also means that promises are not made forever (since Russia promised not to invade Ukraine if they'd give back their nuclear weapons). More importantly, you don't know how long such a promise lasts.
Based on how Russia treated Ukraine we can state:
* Nuclear weapons act as a deterrent. It doesn't matter what treaties and promises are in place. Nuclear weapons will fail as a deterrent if there's a proper defense against it. Currently, there is no proper defense against it. It will be unknown when there will be or if there ever will be.
* Promises and treaties show an intention. That intention may not be acted upon at any given point in time.
We know that North Korea has nuclear weapons. This means that North Korea has a deterrent against any other country including China. Moreover, North Korea has made this deterrent more potent by showcasing it is willing to starve its own population. This means they care less about having nuclear weapons targeted at them than the other way around. To see why this strategy is effective one could read up on the mentality Swiss pikemen in the 15th century [1].
If China invades North Korea, then they better have a way to defend against nuclear weapons. If they don't, then I estimate that they they risk about 50 million to 100 million people directly dying to being severely injured [2], not to mention the nuclear fallout would cause second order effects.
[1] In short: because they were willing to go on suicidal charges against other pikemen of other nations, they became the most feared pikemen to fight against. Fighting against a Swiss unit would mean certain death for both groups of pikemen.
[2] North Korea has about 40 to 50 nuclear weapons (source: a quick Google search). China has 290749208 people in the 40 biggest cities (source: https://en.wikipedia.org/wiki/List_of_cities_in_China_by_pop...). I took a rough 15% to 30% of that.