It's true, as Anne Applebaum says, that China is the only country in the world with any real influence over North Korea. So why do they put up with Kim Jong-il's antics? The usual answer is that they're afraid of pushing too hard lest his regime collapse and send millions of refugees streaming across the border into Manchuria. Applebaum, however, speculates that that isn't it at all. China actually wants North Korea to continue its hotheaded ways:
Despite the risks, there are good reasons for the Chinese to prod Kim Jong-il to keep those missiles coming. By permitting North Korea to rattle its sabers, the Chinese can monitor Obama's reaction to a military threat without having to deploy a threat themselves. They can see how serious the new American administration is about controlling the spread of nuclear weapons without having to risk sanctions or international condemnation of their own nuclear industry. They can distract and disturb the new administration without harming Chinese-American economic relations, which are crucial to their own regime's stability.
And if the game goes badly, they can call it off. North Korea is a puppet state, and the Chinese are the puppeteers. They could end this farce tomorrow. If they haven't done so yet, there must be a reason.
I don't really have much to add to this. It's just that the refugee explanation of Chinese behavior has always struck me as moderately unconvincing, so I'm sort of interested in alternatives — even if they do come wrapped in some variant of "China must be stopped!" fearmongering. Which this one does. But it's worth a thought anyway.