China is North Korea's "ally" in the sense that every wacky little brother has a big brother who rolls his eyes and gives him wedgies when it becomes necessary. I guess that's what you get for clinging to communism. You're stuck with friends like Bowl Cut Jr., although you do get very nice things written about you by Thomas Friedman in the New York Times.
And when it looks like little brother is so out of control that war looms, people think it's your job to stop it. And if China wants to do something, now might be a good time because the Norks still can't get their missiles to launch correctly. Once they get that figured out, any sort of aggressive move against Pyongyang could provoke an attack on Seoul. Maybe Xi Jinping figures there's no time like the present to bring an end to the Kim dynasty: