The Chinese and North Koreans are allies for many reasons, not the least of which is the fact that about a million Chinese died defending the NoKo's during the Korean war. That blood connection has cemented an uneasy friendship that some Chinese would dearly love to change, but that current circumstances make it difficult to do so.

Telegraph:

Although North Korea depends on China for aid and trade, and despite repeated declarations of friendship by both sides, the North Koreans that I knew really did not like the Chinese, whom they regarded as rude, bullying, fond of eating the most disgusting things and with unfortunate personal habits. I once asked a friend whether they minded working with foreigners. They replied that they liked western foreigners, and found them polite and good company. "Not like Chinese", they said, shuddering.

My contacts with China suggest that this dislike is reciprocated. Some have told me that they find North Koreans mendacious and devious, and almost impossible to understand.

But despite this antipathy the two countries have been forced into a strategic embrace. North Korea needs Chinese aid and diplomatic support, and China needs to avoid a North Korean collapse (which might send floods of refugees into China's north-eastern rust belt). It also finds North Korea a useful buffer against US forces in South Korea.

Moreover North Korean soil is soaked in Chinese blood from the Korean War. The families of the perhaps one million Chinese soldiers who died "defending North Korea against imperialist aggression" feel strongly that an alliance defended at such cost should never be abandoned.

There are however many other Chinese who believe that the time has come to end, or at least reduce, China's support for North Korea. If their view prevails then China has the ability to apply overwhelming pressure to North Korea, by halting aid or limiting trade. If it were ever to do this it is likely that there would be fundamental changes in North Korea - the trick would be to achieve these without violence.

That day seems far off as China is refusing to go along with new UN sanctions on North Korea:

A strident editorial in the Global Times, a Communist Party-backed newspaper, said yesterday that while China believed North Korea should shelve its nuclear programs, Pyongyang should not be punished by the US, Japan and South Korea.

Last week, North Korea carried out its third nuclear detonation, testing out an underground nuclear device at a remote site in the country's north. The shockwaves, which were felt in remote areas of China, were initially reported as an earthquake until it emerged a test had been held.

The move was immediately condemned by world leaders and prompted an emergency meeting of the UN Security Council to negotiate what action to take against North Korea.

However, Pyongyang warned that it planned up to two more tests in the next few months.

China is a major contributor of foreign aid to North Korea and has previously said it does not believe the nation's impoverished population should be harmed by US sanctions.

"China is facing a difficult diplomatic problem. Its only choice is to avoid worsening the situation," the Global Times editorial said. "China opposes North Korea's nuclear tests and should express its opposition clearly through actions despite Pyongyang's discomfort. China should inform Pyongyang that if it continues to conduct nuclear tests, assistance will be further reduced."

However, it also said China would not directly back the world's response in punishing North Korea. "But China is still North Korea's friend," it said.

"This means China won't join hands with the US, South Korea and Japan to blockade North Korea on land and sea and will oppose any UN resolutions that threaten the North Korean regime. China opposes North Korea possessing nuclear power, but won't see a sharp turn in its attitude to Pyongyang."

China can't afford a more impoverished North Korea as the refugee problem is escalating because of food shortages, and the North Korean economy totters as a result of massive spending on its nuclear program. If China can be shown an "out" of their relationship with Pyongyang without massive relocation, they may jump at it.

But any scenario that involves taking out the leadership in North Korea includes, by definition, civil unrest. China can't afford that and so it will continue to support the North Koreans diplomatically.