It seems North Korea has finally gone too far — even for China, its patron state and only true friend. For the first time, Chinese leaders seem to be taking modest steps intended to punish their southern neighbor for threatening to conduct a third nuclear-weapons test.

After a year in office, North Korea’s chubby, naif supreme leader, Kim Jong Un, has remade himself into a belligerent bully, openly threatening South Korea — and the U.S.

My question is: With what?

North Korea has already conducted two nuclear tests — both of them duds, nuclear experts say. Now that it’s threatening a third one, South Korean government officials say the preparations are advancing rapidly. In fact, a few days ago, Yonhap, the South Korean news agency, reported that the North Korean military has spread a tarp over the opening to the underground testing site, to keep spying satellites from peering inside.

But the most important development is China, which has never gone much beyond mild public criticism of its belligerent neighbor. The two states’ relationship has been testy.

North Korea accedes to China’s heavy-handed and condescending patronage in part because China supplies North Korean leaders with jewelry, fine wine, perfume, yachts and other luxury goods beyond imagining — as well as bundles of cash. Sometimes the North Korean air force flies to southern China for takeout McDonald’s Big Macs.

As for China, its leaders worry that if North Korea’s government does fall, tens of thousands of refugees would flood across the border. At the same time, South Korea, a close U.S. ally, would likely take over the territory.

While the two states don’t like each other, they are dependents nonetheless. But even for the Chinese, these nuclear tests seem to be more than they can bear. Over and over again the Chinese foreign ministry has insisted that the Korean peninsula must remain nuclear-free — obviously afraid South Korea, working with the U.S., would respond tit-for-tat.

So in recent days China voted in favor of a United Nations Security Council resolution, condemning North Korea. And the government’s nationalist mouthpiece newspaper, the Global Times, wrote that “if North Korea engages in further nuclear tests, China will not hesitate to reduce its assistance” to the North.

We’ll see. That hasn’t worked so far. But the China bureau of Nikkei, the Japanese news service, reported that “China is considering imposing new financial sanctions on North Korea, including freezes on assets at North Korean bank branches in Beijing, to deter Kim Jong Un’s regime from conducting another nuclear test.” It added that, already, China has “stiffened inspections of North Korea-bound shipments at customs checkpoints along their border.”

The International Crisis Group made the same observation. Now, is that going to cripple North Korea? Of course not. China hasn’t taken actual steps like this before. So I view this as significant.

North Korea’s reaction to all of this? More bluster and blunder. The North Korea Defense Commission released a statement, saying: “We do not hide that a variety of satellites and long-range rockets which will be launched” by North Korea “one after another, and a nuclear test of a higher level will target the U.S., the sworn enemy of the Korean people.”

U.S. intelligence has said North Korea’s missiles could at best reach Hawaii — bad enough. But Kim Jong Un also knows that any sort of attack would prompt the U.S. to “turn North Korea into a parking lot,” as Condoleezza Rice once put it when she was secretary of state.

But now we can hope that China, with a new leader, will finally step up. For a decade, Western nations including the U.S. have been pushing China to restrain its neighbor. During all that time, China’s leaders have done nothing more than offer lip service to those entreaties.

Now, at last, the new leader, Xi Jinping, actually appears to be angry. He’s the only one who can actually restrain his neighbor. No other state has any real influence with Pyongyang.

Let’s hope that China’s small, tentative steps to chastise Kim Jong Un evolve into a strategy that actually makes a difference.