The reasons are both ideological and historical. First, China’s main interest in North Korea is not denuclearization; it is ensuring that the North Korean government does not fall. While Beijing might be exasperated with the Kim dynasty’s uncanny ability to wag China’s dog, China will support Pyongyang because the alternative, a North Korean collapse, is worse. While many South Koreans fear the cost of unification with their brothers to the north, China opposes that even more stridently.

Hundreds of thousands of refugees would pour into neighboring China. Then China would have to determine how to deal with South Korean and U.S. troops who would move to secure the North’s nuclear weapons. Beijing would also be faced with millions of Korean-Chinese inspired by a new, united homeland. The issue of a potential North Korean collapse is so sensitive that Chinese officials have declined repeated U.S. entreaties to discuss scenarios of how to avoid clashes when and if it happens.

Clearly for Beijing, the presence of a Communist buffer state, even an irritating one, between China and South Korea remains critical. A Korean Peninsula united under the South would pose a huge challenge to China’s political system. East Germany is the parallel some Chinese use when asked why China won’t squeeze Pyongyang: The Soviet Union collapsed when the Berlin Wall fell. If the no-man’s land separating North and South Korea were breached, could the same thing happen to Beijing?