Based on his experience doing business with a few Chinese bankers, Donald Trump feels comfortable stating unequivocally that China is in complete control of events in North Korea, and can change the regime’s behavior at a moment’s notice. As with most things, he is clueless on this subject. The relationship between China and North Korea is complex and fraught.
The Chinese government decided a long time ago, and with substantial reason, that while the North Korean regime was dangerous and irresponsible, every alternative to it was even worse. The North Koreans know this, and consequently pay little attention when the Chinese try to tell them what to do.
While the nature of the regimes is obviously very, very different, the China/North Korea relationship reminds me a bit of the relationship between the US and the Netanyahu government, which knows it has enough support in Congress and with the American public to ignore any attempt by Obama to leverage aid and diplomatic support for moderation in the West Bank and Gaza.
The US is often accused of trying to encircle China, but in the case of North Korea, we would welcome any developments that would increase Chinese influence. Unfortunately, any North Korean official who becomes too cozy with the Chinese is viewed as a threat to the regime and is likely to wind up being executed. A moderate North Korean regime under Chinese control is, therefore, not likely in the foreseeable future.