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.
Another consequence is that North Korean leaders are likely to be even more suspicious of China's motives in the future and be even more demanding of proof that there is no change in Chinese policy. As a result, in the short term, China may be even more deferential to North Korea, as it was when it sent its entire top leadership to Changchun to greet Kim Jong-il in August.
Chinese officials realise Pyongyang's paranoia and have declined to take any action that might indicate anything less than total dedication to the perpetuation of the Kim dynasty in North Korea. Thus, they have brushed aside American suggestions of discussions of possible contingency plans to handle a collapse of the North Korean regime.
One WikiLeaks cable reported that President Hu Jintao was asked point-blank by South Korean President Lee Myung-bak in January 2009 'what China thought about the North Korean domestic political situation and whether Beijing had any contingency plans'. The Chinese leader ducked the issue by pretending not to hear the question.
The six-party talks were a definite plus for China, providing a platform for showing off its diplomatic prowess and substantially improving its relationship with the United States.
But now that North Korea has succeeded in developing nuclear weapons, China can no longer simply call for a resumption of the aborted talks as though it is somehow an uninvolved party. By supporting North Korea, China has disqualified itself as an impartial party.
It must realise that its behaviour has consequences, manifest now in the changing political alignment in northeast Asia, where South Korea and Japan are tightening their relations with the United States. This is certainly not in China's interests.
North Korea's actions have put it beyond the pale. Beijing must realise that North Korea is now an albatross around its neck and will drag it down unless China acts in its own long-term interests.
Frank Ching is a Hong Kong-based writer and commentator