China seems to delight in letting North Korea make trouble for the US. I have news for China. Your lenience toward North Korea will have consequences that the US cannot prevent and that you will not like.
Consider that Japan is watching the US. She is judging the steadfastness of the US posture in Afghanistan and Iraq. She remembers the way the US ran from Vietnam. She believes that even Israel does not trust the US nuclear umbrella for protection. If Japan sees that she must live with North Korean nuclear-tipped missiles, she will almost certainly decide that the prudent thing would be to have some of her own.
And so, I think that the US should begin walking away from North (and South) Korea. Either China will step up to the plate and bring this bit of proliferation madness to an end, or China will have to live with a nuclear armed Japan. And possibly (in time) even a nuclear armed South Korea.
Let's see. Nuclear India. Nuclear Pakistan. Nuclear North Korea. Nuclear Japan. That will be quite a nuclear neighborhood for nuclear China to handle. We should wish them luck.