China’s North Korea Problem
“Beijing has long viewed North Korea as both a dangerous liability and a strategic asset. Above all, China fears that its unpredictable and often aggressive client state could spark a military conflict with South Korea and the United States, potentially dragging Beijing into a second Korean War. Even short of provoking outright conflict, North Korea’s advancing nuclear and missile capabilities still threaten to trigger a regional arms race. The United States and its allies have already expanded missile defense deployments in East Asia, which Beijing views as a direct threat to its strategic deterrence capabilities. Although China retaliated against South Korea for deploying THAAD in 2016, Chinese state media acknowledged that North Korea bore some responsibility. A 2017 commentary in China’s state-run Global Times, for example, blamed Pyongyang for giving Washington and Seoul a justification for THAAD deployment and argued that the North’s provocation had harmed China’s strategic interests.”
“Beijing is also acutely aware of the risks posed by its shared border with North Korea. Pyongyang’s continuing nuclear and missile developments could invite new rounds of international sanctions, potentially crippling North Korea’s already fragile economy and causing a refugee crisis in northeast China. Nearly half of North Korea’s known major nuclear sites lie within 30 miles of the shared border, making any nuclear mishap—whether caused by an accident in North Korean operations or a preemptive U.S. strike—a possible environmental disaster for China. Beijing’s worst strategic nightmare is a unified Korean Peninsula under Seoul’s leadership, with a continued U.S. military presence, which a sudden collapse of the North Korean regime could produce.”
“But North Korea also serves China’s strategic interests in two important ways—both with respect to Taiwan…”
https://www.foreignaffairs.com/china/chinas-north-korea-problem