But in a recent interview, Trump acknowledged that the issue is, well, complicated. In a conversation with the Wall Street Journal, the president said he came into his first meeting with Xi convinced that China could curtail North Korea’s nuclear threat. Xi had to explain Chinese-Korean history to Trump, who then realized something important: “After listening for 10 minutes, I realized it’s not so easy,” he told the Journal. “I felt pretty strongly that they had a tremendous power North Korea. … But it’s not what you would think.”

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Trump's comments come as North Korea’s Kim Jong Un amps up his provocations. Analysts say the regime may conduct another nuclear missile test Saturday, timed around the most important day on the North Korean calendar: the anniversary of the birthday of its founder. As Anna Fifield reported this morning, “recent satellite images of North Korea’s underground nuclear test site, at Punggye-ri in the northeast of the country, shows it is 'primed and ready,' according to the 38 North website, which follows North Korean affairs. People and vehicles continue to move around the site, according to images from Wednesday, the report said.”

Last night, Japan's prime minister raised the prospect of a chemical weapons attack. South Korea's military said it would “mercilessly retaliate.”

Trump is right in thinking that to really pressure North Korea, he needs China's help. And he's right about something else, too: It's complicated. As USA Today put it, “China is North Korea's neighbor, protector, chief trading partner and economic lifeline.”

The fates of the two nations have been entwined since the end of World War II, with the establishment of the communist states. They have a long-standing mutual aid agreement. And their bond was cemented during the Korean War, when Beijing sent troops across the Yalu River to push back U.N. forces.

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While Japan, South Korea and the United States are increasingly concerned by North Korea's nuclear arsenal and missile program, Beijing's nightmare scenario is a collapse of the Kim regime.

China would have to step in to support any government that replaces Kim or risk a total economic collapse in its neighbor. North Koreans, who have already gone through food shortages under the Kim regime, could become refugees in such a scenario.

A collapse of the Kim regime could also open the door to reunification with South Korea, another nightmare for Beijing. A reunified Korea allied with the United States would potentially be a powerful regional rival.

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Some in China even worry that the threat might be existential. As longtime Washington Post correspondent John Pomfret writes, “East Germany is the parallel some Chinese use when asked why China won’t squeeze Pyongyang: The Soviet Union collapsed when the Berlin Wall fell. If the no man’s land separating North and South Korea were breached, could the same thing happen to Beijing?”

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Chinese companies also benefit from a relationship with North Korea. The rogue country's weapons program relies heavily on foreign parts. And those parts are acquired from businesses based in China, experts say. According to a report released last month by the United Nations, Chinese companies are privately (and illegally) shipping products to North Korea. It's not clear whether the Chinese government tacitly approved those exports.

“There’s all kinds of slack in the system,” Joshua Pollack, a former consultant to U.S. government agencies on arms control and a senior research associate with the James Martin Center for Nonproliferation Studies, told my colleague Joby Warrick. “It could be that the Chinese don’t care enough to do much about it. A second possibility it that they don’t have the systems — such as strong export controls — in place. Or that it’s just corruption.”

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It also doesn’t help that China sees North Korea as a useful thorn in America’s side.

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But while the Chinese Foreign Ministry continues to describe China and North Korea as “friendly neighbors,” there are signs that the relationship is fracturing. Last year, China signed onto an international agreement that severely sanctioned North Korea. In February, China suspended coal imports from North Korea, a move that coincided with a North Korean tirade against China, accusing it of “dancing to the tune of the U.S.” and “styling itself as a big power.”