Updated on July 29 at 4:45 p.m.

The President of South Korea, Moon Jae-in, has asked the U.S. to open negotiations that would allow South Korea to build more powerful ballistic missiles in order to counter an increasingly aggressive North Korea. On Friday, the government in Pyongyang fired an intercontinental ballistic missile (ICBM), its second this month, that flew about 45 minutes straight up and crashed into the sea off Japan some 620 miles away. U.S. national security advisor H.R. McMaster confirmed early Saturday that Washington had accepted Moon’s offer, and would begin negotiations shortly.

The decision is a remarkable change of direction for Moon, who came to office just two months ago. Moon campaigned on a platform that favored dialogue with North Korea. He’d also opposed the U.S. missile defense system, called THAAD, that was aimed at countering a North Korean threat. But on Saturday Moon asked his government to work with the U.S. to temporarily deploy the full THAAD system. The move will likely upset China, which says the missile defense system could be used to spy into its territory and that it would escalate tensions with Pyongyang.

China’s foreign ministry, in a statement early Saturday meant to scold North Korea, urged its leaders “to respect United Nations security council resolutions and stop all acts that could worsen tensions.” Then, after learning of Moon’s request to Washington, China issued an even more pointed statement, saying that “THAAD won’t solve South Korea’s security concerns, won’t solve the related issues on the Korean Peninsula and will only further complicate issues.”

North Korea’s leader, Kim Jong Un, warned in January that his country was close to testing an ICBM that could deliver a nuclear warhead to the U.S., and many experts believe this is now possible—or at least it could be shortly. In its first test this month, North Korea launched a Hwasong-14 missile into the atmosphere on an arc that took it many times higher than the orbit of the Space Station, to a landing some 600 miles away. As my colleague, Uri Friedman pointed out, experts analyzed the rocket stages and capacity and believe that, on a different, flatter arc, it would be capable of flying many thousands of miles, theoretically placing U.S. cities like Los Angeles, Denver, Chicago, possibly even New York City within striking distance. In making good on that threat, one of the last challenges for North Korea is “reentry technology.” This is the challenge of allowing warheads to endure the heat generated on descent back through Earth’s atmosphere. Future tests might demonstrate how far North Korea has come in this regard.

Moon Jae-in’s request to allow South Korea to build more powerful missiles would require re-examination of agreement from the 1970s between South Korea and the U.S. According to the treaty, the U.S. would provide help in building the missiles. In exchange, South Korea agreed that the missiles would be limited to a range of less than 500 miles, and with payloads no heavier than half a ton. The intention was to limit an arms race in the region. Moon now says that, with North Korea’s new capabilities and aggressive stance, his own country needs more protective firepower. An increase in range probably doesn’t matter: all of North Korea is within the current 500-mile range. Therefore South Korean officials will likely ask to increase payload, so as to ensure that South Korea could destroy underground silos and locations of Pyongyang’s leadership. Any increase, however, is likely to upset China.

The Trump administration has grown increasingly frustrated with Beijing, which is North Korea’s largest trading partner and only significant protector. On Saturday, Secretary of State Rex Tillerson blamed China and Russia for allowing Pyongyang to reach this point. “As the principal economic enablers of North Korea’s nuclear weapon and ballistic missile development program, China and Russia bear unique and special responsibility for this growing threat to regional and global stability,” Tillerson said. The Trump administration has pushed China to increase its pressure on North Korea, and has placed sanctions on a small Chinese bank for its operations in North Korea. But Beijing has been reluctant and has argued for years that its leverage, while greater than any other country’s, still is limited.

At the United Nations Security Council, the U.S. is pushing both Russian and China to enact much tougher sanctions against North Korea, which has provoked the predictable, histrionic response from Kim Jong Un’s regime. In a statement released shortly after Friday’s launch, North Korea’s official news agency, KCNA, said, “If the Yankees brandish the nuclear stick on this land again despite our repeated warnings, we will clearly teach them manners.”