Since the 1950s, when Chinese and North Korean troops fought side by side against South Korean, American and other forces fighting under the UN flag, Beijing has described the bilateral relationship as being 'as close as lips and teeth'. This expression stems from events some 2,600 years ago, before the first emperor unified China, when the state of Yu was asked by the state of Jin to co-operate in an attack on the state of Guo.
One Yu minister warned: 'Yu and Guo are like teeth and lips; when the lips are gone, the teeth will be cold. When Guo is annihilated, Yu will be the next victim.' Sure enough, after Jin vanquished Guo, Yu was conquered too.
China has for decades seen North Korea as its lips, providing a buffer against the anti-communist government in Seoul and its ally, the United States. But while this may have been valid in the past, it is certainly not the case today. In the case of North Korea, China seems to be caught in a time warp.
South Korea is no longer an enemy; China is now South Korea's most important trading partner. From an economic standpoint, South Korea is far more important to China than North Korea. And North Korea, instead of providing protection for China through its provocative actions - such as the testing of nuclear weapons and missiles and, most recently, the shelling of a South Korean island - is proving to be a liability.
The publication of leaked diplomatic cables by WikiLeaks provides a little more texture. They tell us that some Chinese officials have serious misgivings about North Korea. But it is going too far to say that Beijing is willing to accept a unified Korean Peninsula under Seoul's control.
In fact, all indications are that China continues to support North Korea. It has not reprimanded North Korea for the latest attack but instead urged both Koreas to exercise restraint.
The WikiLeaks disclosures are likely to embarrass Beijing, as they do the US.