Such a system cannot be productive in the long term, and their economy started to crumble. It would make the regime unstable. It has to find a way to survive. Unlike other communist countries, which embraced political and economic reform, N. Korea did the exact opposite. It saw how things could go horribly wrong if they take the road of reform. Especially it was deeply traumatized by what happened to Ceausescu regime in Romania, a close approximation of N. Korean regime. So N. Korean regime further tightened its grip, and looked for the ultimate insurance against regime change: nuclear weapons. So they poured much of their economic resources to developing nukes and missiles, their delivery system. They believe this is the only way to perpetuate the rule by hereditary cult of personality.
So N. Korean regime created two sources of instability: 1) crumbling economy 2) nuclear threat. It tried to use the latter to offset the former. With nuclear blackmail, it receives generous economic aids, and keeps the economy from free fall. This is predicated on the aid providers’ hope that N. Korea would eventually give up its nuclear programs. However, N. Korea has no intention of doing it. It believes the nuke is the only guarantee against its downfall. It does not want to be Ghaddafi’s Libya or Saddam's Iraq. When and if the regime is threatened by uprising or coups, it should have a means to thoroughly check external influence. Beside, its state ideology Juche dictates that the regime should be seen as standing up to mighty powers. Actually, what it wants most is the regime guarantee by U.S., a peace pact which recognizes its legitimacy over entire Korean Peninsula.
China values N. Korea as useful buffer against pro-West S. Korea and U.S. It wants N. Korea to be a friendly ‘stable’ communist country. China wants N. Korea to be economically productive and stable, and refrain from periodic military adventures, which could invite the attention of its adversaries such as U.S.
Until recently, China put up with N. Korea. It kept N. Korea alive by providing steady stream of economic aids. However, the level of military destabilization is getting worse not better. It conducted nuclear tests, and claim that it possessed nuclear weapons. They are working on ICBM, too. It is constantly providing reasons for large U.S. military presence in its neighborhood.
China tries to talk them out of it, but N. Korea is not interested. N. Korean regime does not want to be transformed into another ‘China’ ruled by a collective leadership where people no longer worship a god-like ruler from a sacred family. Economic reform means the end of dynasty. They believe that following China's advice will lead to their demise.
They are intent on completing their quest for nuclear arsenal. No amount of food or money can make them stop. China had enough of this. However, they are not about to take drastic action such as complete embargo or military invasion. Such actions entail great risk, and it is far from certain that things will turn out the way they envisioned. So instead they tried to nurture pro-China faction in N. Korea. It seemed to be going well, until Kim Jong-un executed its leader, his uncle Jang Sung-taek, and unleashed the purge of entire faction. Now there is no ally of China inside N. Korea. Kim Jong-un knows that, if he goes to China, he will be under great pressure from Chinese leadership to change his way. Kim figures that there is no friendly audience in Beijing. Besides, there is a security concern. Something can happen during his trip. Attempts at his life or a coup back home while he is away, even if such a possibility is low.
Under the current situation, Kim is backed into a corner. China in effect drew a line. No more nuke and belligerence. S. Korea is not pliable as it used to, either. Gone are the days of “Sunshine Policy.” Once, such a policy enjoyed a popular support. Not anymore. In S. Korea, these days, it takes some courage to openly advocate appeasement policy in public. They get roundly booed.
So Kim Jong-un has little maneuvering room. He sent out feelers to Russia, but was met with skepticism. Every country around N. Korea is souring on him and now trying to find some common ground against N. Korea. Kim Jong-un is a product of cult of personality in a sealed totalitarian country, even though the seal is starting to break down. He cannot be Deng Xiao-ping. He is more like third generation Mao Zedong. That is why he is compelled to resist China's way.
PS: You are right that Chinese communists did little to nothing in fighting Japanese. Nationalists did all the work.
What is your recommendation concerning the exposure of our troops along the DMZ? Should they be pulled back out of the range of artillery in the initial onslaught? Should they be withdrawn from the peninsular altogether? Should they be left in place as a necessary tripwire?
What about the South Koreans, are they going to pay their own way, ensure their own defense, contribute to the general coalition that should be built to contain China?
