North Korea is hailing a “successful” fifth nuclear test, which it carried out Friday morning local time.

The device tested, which created a 5.3-magnitude tremor at its Punggye-ri test site, was reportedly in the 20- to 30-kiloton range, much more powerful than the North’s previous detonations. The last test, in January, yielded only about seven to nine kilotons.

The North Koreans had been ready to test this device since May. So why did they wait until now? Some are suggesting the detonation celebrated North Korea’s Foundation Day, marking the 68th anniversary of the establishment of the Democratic People’s Republic of Korea. But from all indications, the Kim regime tested at this time because it realized China would not impose costs for the detonation.

The test took place three days after Pyongyang’s nuclear envoy traveled to Beijing. Choe Son Hui, deputy director general of the Foreign Ministry’s U.S. affairs bureau, arrived in the Chinese capital on Tuesday.

We don’t know what Choe—who was deputy chief envoy to the six-party denuclearization talks, which have been dormant since 2008—and her interlocutors said this week. Nonetheless, it was evident that the North Koreans were confident of the Chinese reaction.

At the moment, Beijing is far more upset with Seoul than Pyongyang.

In July, South Korea and the United States announced they would deploy the American-made Terminal High Altitude Area Defense system on South Korean soil. Beijing is worried that THAAD’s high-powered radars will reach into China and could help the U.S. shoot down Chinese missiles. Washington denies that is the case and has been willing to share technical information, but Beijing has not been mollified.

Since the announcement, Beijing has taken a number of steps to snub the South diplomatically and undermine its economy.

With Beijing upset at Seoul, the North Koreans evidently think they can do what they want. On Monday, the North launched three medium-range, nuclear-capable Nodong missiles. The tests, on the second day of the China-hosted G20 summit, were conducted right after Chinese ruler Xi Jinping met on the sidelines of the event with South Korea’s president, Park Geun-hye. Clearly, Kim Jong Un was not worried that China would react unfavorably to the launches.

It’s clear the North Koreans know that as a general matter they have Chinese support. Trade across the Sino-Korea border is booming at the moment, an indication that Beijing is not enforcing Security Council Resolution 2270, the fifth set of UN sanctions on the North’s weapons programs.

Moreover, some of the traded items are clearly destined for Kim’s military. China, according to David Albright of the Institute for Science and International Security, did not interrupt the flow of materials and components for Pyongyang’s nuclear weapons program, such as cylinders of uranium hexafluoride. Also allowed in, worryingly, were vacuum pumps, valves, and computers.

The North Koreans know that Xi sees the U.S. as China’s main adversary, blocking Beijing’s ambitions in almost every direction. That’s probably why President Obama and National Security Adviser Susan Rice got a rough reception on Saturday in Hanghzou as they arrived for the G20. Kim, seeing how Xi treated Obama, thought he could get away with delivering his own radioactive-laced snub.

Kim knows that Xi is not about to further goals, like the denuclearization of North Korea, that Washington promotes, and so Pyongyang thinks it has a big green light in its quest to possess the world’s most destructive weapons.

Pyongyang will make fast progress in developing nukes—until the U.S. and the rest of the international community realize they have a China problem as much as a North Korean one.