Liu Jieyi, China’s ambassador to the United Nations, addresses a meeting on North Korea at U.N. headquarters in New York on Wednesday. (Jason Szenes/European Pressphoto Agency)

SECRETARY OF State John F. Kerry emerged frustrated from a meeting with China’s foreign minister in late January after proposing new U.N. sanctions on North Korea. Beijing balked, saying it was not willing to take steps that risked destabilizing the regime of Kim Jong Un even after the regime conducted what it claimed was a hydrogen bomb test. On Wednesday, China seemingly reversed course, joining a unanimous U.N. Security Council in imposing the toughest sanctions applied to North Korea in more than a decade.

What prompted this welcome change? Mr. Kerry and his State Department team spent weeks negotiating with their Chinese counterparts — and North Korea’s launch of a long-range rocket last month over Beijing’s objections may have spurred a U.S.-Chinese convergence. Our guess, however, is that China’s switch had a lot to do with steps taken by South Korea and Congress.

In Seoul, the government of President Park Geun-hye, which Beijing has been courting, decided to move forward on plans for deploying a U.S. missile defense system that China regards as a threat. Meanwhile, Congress adopted new U.S. sanctions that could penalize Chinese companies and banks that do business with North Korea. In other words, the Chinese leadership finally was forced to consider tangible consequences for its coddling of the reckless and increasingly dangerous North Korean ruler.

The result is sanctions that, on paper, could have the most damaging impact in Pyongyang since the George W. Bush administration succeeded in locating and freezing the regime’s foreign financial assets in 2005. The new resolution orders the inspection of all cargoes entering and leaving North Korea, bans its export of some minerals and import of arms, and mandates a shutdown of its international banking activities. It also cuts off supplies of most aviation fuels and expands the list of luxury items the elite cannot receive.

What remains to be seen is if Chinese President Xi Jingping will apply these sanctions rigorously — or whether his consent to them was merely a feint intended to head off unwanted developments such as South Korean missile defense. It’s telling that China’s U.N. ambassador brought up the missile defense system just before voting for sanctions — and that a first U.S.-South Korean meeting on the missile defense system was postponed as the resolution came together. Also revealing is a new report by a committee of U.N. experts, which found massive evasion of past U.N. sanctions against North Korea, with the connivance of Chinese companies and banks as well as regimes such as Burma’s.

For China, the best-case scenario is that the new sanctions hurt just enough to prompt the Kim regime to offer to negotiate on its nuclear arsenal, something it has refused to do in recent years. That would shift the burden of containing the regime to the United States, without risking its collapse. Though the long history of such negotiations is one of repetitive failures, the Obama administration likely would embrace such an offer. On the other hand, if North Korea’s regime proves intransigent, China could be tasked with overseeing a prolonged economic siege. The chances that it will do so still seem slim.