China has announced that if the US launches a preemptive strike they will retaliate. However, if North Korea attacks the US then they are Green Lit.
China was seriously considering regime change in North Korea earlier this year. The Kim Family Regime has gone from a barely tolerable criminal enterprise to an active embarrassment. The DPRK is pain, a bother, a nuisance and worst of all to China a destabilizing element.
The thing you have to remember about the Han is that war is not glorious for them because all of their wars have been either civil wars or foreign invasions. War has always meant maximum suckage to no benefit, if you were Chinese. They were never the victors because you don't win those kind of wars you just survive them. If war was necessary make it quick and do the minimum of damage necessary to end the hostilities.
That is what Sun Tzu's the Art of War is really meant to accomplish. Here is how you win fast with the minimum of damage to everyone.
It's why China jumped into the Korean War with both feet in October of 1950. The Communist regime in China was only three years old. The Americans were openly committed to restoring Chiang Kai-shek to power. With the North Korean regime on the brink of annihilation there would be no buffer state between China and an American client state that would be a safe harbor for Chiang's supporters to strike from. War was clearly and obvious inevitable, so it was better if it was on the Korean side of the border and not on Chinese soil.
When a stalemate was successfully established the end result was two Koreas.
The South was the client of one state. It was in the best interest of the United States to build up South Korea. Which we did.
Sadly for the NorKs, their friends and allies had almost as many problems with each other as they did with the America.
As you can see on the map below North Korea is equally useful in both threatening Vladivostok as well as cutting off the south Yellow Sea from China.
Rather than build up a client that could be powerful enough to start playing off one hostile ally against another. China and Russia quietly agreed to let North Korea barely survive as an agrarian backwater and let the wonders of Communism do the rest for keeping them impoverished.
When the Soviet Block collapsed, the troubles began in earnest for the Kim Regime.
China still didn't want an American client on it's border. And America wanted minimal conflict on the Korean peninsula, SouthEast Asia holding no magic memories for us.
Both Korean governments were committed to reunification at first. But after sixty five years of separation each as been Othered from each other. Pretty easy to do with Korean cultural attitudes.
The North no longer has the option of conquering the South. Not with their weapons, strategy and tactics. They are a Second Generation military facing a Third Generation military that would have total and complete air dominance from day one. A terrain that drastically favors defense is the icing on the cake.
The Kims could only watch impotently as the South went from strength to strength, while they were being provided with a bare minimum of support from their supposed allies. The Soviets and the Chinese held that while World Communism was of course the future, the Kim family itself didn't have to part of that future and could be replaced at need. The Kims didn't like that idea, not even a little bit. So they opted for the only path to survival available to them, the Puffer Fish Strategy. Inflate everyone's idea of how dangerous you really are. All you need to do is be an aggressive drama queen and given Korean culture that is pretty easy.
Today, the South does have the option of conquering the North at will but there is a hell of a price tag for that. Seoul will by subjected to the biggest terror attack in history. Artillery, Chemical weapons and terror troops emerging from tunnels. Although the last assumes that the North still has a secret Tunnel of Aggression or two that hasn't been found.
And once the South wins, look at what they are stuck with. It would be like America suddenly inheriting a land mass as poor as Haiti but with a population of 300 million.
So a new but degenerating equilibrium was established. For the Kims hanging on to power is literally a matter of life an death. If they fall they will be dragged screaming from their beds and they know it. So they rely even more on the Puffer Fish Strategy than they ever have before, "I'M CRAZY MAN! DON'T MESS WITH ME! I'M NUCLEAR CRAZY!" And then accept various offerings of material aid to pipe back down again.
Their entire survival depends on appeasement. The problem they now face is what happens when they run into a world leader they can't scare?