South Korean President Moon Jae-In has stirred up a decades-old dispute with Japan, a move that risks disrupting the international effort to contain North Korea.

For years, Seoul and Tokyo have been at loggerheads over the matter of comfort women — the thousands of girls and women, many of them Korean, who were forced to work in Japan’s military brothels during World War II. The two countries reached an agreement in 2015, in which Tokyo shelled out $8.8 million to a fund for victims, but Moon on Thursday criticized that deal, saying it “cannot solve the comfort women issue.”

His comments came after a South Korean panel concluded that the 2015 treaty failed to meet victims’ demands for compensation. In response to the panel’s findings, Japanese Foreign Minister Taro Kono warned that any attempt to revise the agreement “would make Japan’s ties with South Korea unmanageable.”

The matter is a topic of national importance in South Korea, where protesters last week called on the government to nullify the deal, and considered crucial to Moon’s populist base.

Seoul and Tokyo are both important U.S. allies and play critical roles in containing Pyongyang’s nuclear and missile program. If Moon allows the comfort women issue to dominate his relationship with Japanese Prime Minister Shinzo Abe, that could hamper international efforts to rein in the nuclear belligerence of North Korean leader Kim Jong Un.

“We will see a fracture develop in terms of U.S.-South Korea-Japanese cooperation in dealing with the North,” explained Stephen Nagy, a senior associate professor at Tokyo’s International Christian University and distinguished fellow at Canadian think-tank The Asia Pacific Foundation.

Moon’s tough stance toward Tokyo contrasts with his conciliatory tone toward Pyongyang. His administration on Tuesday offered high-level talks to its northern neighbor after Kim said that he was open to sending a North Korean delegation to the upcoming Winter Olympics.

“Pressing Tokyo on the comfort women issue while attempting to engage Pyongyang has the risks of alienating Tokyo from Seoul,” Nagy warned.

Kim’s missile tests, which violate U.N. resolutions, remain a global worry, especially in light of recent hostile rhetoric between the dictator and President Donald Trump.

Tensions between Moon and Abe could also boost Seoul’s relationship with Beijing, which is currently on shaky footing amid tensions over Seoul’s decision to install an American-made missile defense system on its soil.

China has its own issues with Japan over the latter’s wartime atrocities, so “by antagonizing Tokyo, Seoul pleases Beijing,” said Leonid Petrov, a Korea expert at The Australian National University.

That, in turn, could bother Washington: “This will inevitably undermine the regional security posture of the United States that traditionally dwells on the anti-communist unity of South Korea, Japan and Taiwan,” Petrov continued. In sum, Moon’s action “creates serious problems for the U.S. security architecture in northeast Asia,” he added.

Nagy, on the other hand, doesn’t expect Beijing will support tensions between Moon and Abe.

China will be wary of South Korean efforts to alienate Japan as it could push Tokyo closer to the White House, he said. That would be an undesirable scenarios for Chinese President Xi Jinping, who doesn’t want increased U.S. influence in Asia.