South Korea has a new president. He and Trump see North Korea very, very differently.

South Korea just elected a new president who is less hawkish on North Korea than Donald Trump.

Moon Jae-in of the liberal Democratic Party is a career human rights lawyer and the son of North Korean refugees. He’s pledged to review his predecessor’s decision to allow the US to deploy the THAAD missile defense system in South Korea and said he wants to improve relations with North Korea, including reopening a joint industrial park on the Northern side of the border that the previous president had said was funneling money to Kim Jong Un’s regime in Pyongyang.

That stands in stark contrast to the Trump administration’s far more aggressive stance toward the dictatorial regime in Pyongyang. Fearful that North Korea is rapidly developing missiles capable of hitting mainland America, the administration has sent some of the US Navy’s most powerful warships to South Korea, and top administration officials are openly talking about a potential preemptive military strike on North Korea’s nuclear facilities. They also sped up the deployment of the THAAD system, currently in place on a South Korean golf course.

But despite — or perhaps because of — the very real threat North Korea poses to the South, many South Koreans favor a gentler approach toward Pyongyang. After all, South Korea’s capital city, Seoul, is within direct firing range of thousands of pieces of North Korean artillery already lined up along the border. As my colleague Alex Ward has written, a 2005 war game predicted that a North Korean attack would kill 100,000 people in Seoul in the first few days alone.

There is also robust opposition to THAAD in some parts of South Korea over safety and environmental concerns as well as fear that China, which staunchly opposes THAAD, will inflict severe economic punishment on South Korea in response to its deployment.

With Moon — whose victory ends nearly a decade of conservative rule in South Korea — now coming into office, THAAD’s future, as well as the future of the US-South Korea relationship, is uncertain.

And given all the turmoil that’s been happening on the peninsula in recent months, from the possibility of a new North Korean nuclear test to the scandal that led to the impeachment and arrest of Moon’s predecessor, more uncertainty is just about the last thing anyone needs right now.

The deployment of the US missile-defense system was approved back in July 2016 in an agreement between the Obama administration and then-President Park Geun-hye. During the campaign, Moon made clear he didn’t like the plan, and called for halting the deployment of THAAD “until the new president takes office and can evaluate its benefits and drawbacks.”

“The delivery should be halted even how, and the next administration should ultimately decide this issue,” his campaign manager said on April 26, less than two weeks before the country’s election.

But why in the world would South Koreans object to a missile-defense system designed to protect them from incoming missiles from the North?

The first reason is that China, one of South Korea’s most important trading partners, is livid over the deployment of THAAD, which it sees as a potential threat to its military capabilities. And the specter of Chinese economic retaliation against South Korea is making some in South Korea reconsider THAAD.

“I want to emphasize again that China is firm in its resolve to oppose the deployment of THAAD in [South Korea] and will resolutely take necessary actions to safeguard its own security interests,” Chinese Foreign Ministry spokesperson Geng Shuang said at a press briefing in Beijing in late February.

“Any consequences entailed from that will be borne by the US and [South Korea]. We strongly urge relevant parties to stop the deployment process and not to go further down that wrong path,” he said.

And indeed, China has already begun to impose some of those “consequences” on South Korea — or, more specifically, on one of South Korea’s biggest companies. The company in question is the Lotte Group, a multinational conglomerate headquartered in Seoul. Back in February, it agreed to give up a parcel of land it owned (part of which is a golf course) to the South Korean government to use as a base for the THAAD system.

Then, all of a sudden, at least 23 Lotte Mart stores across China were mysteriously shut down by Chinese authorities. As CNN reports, Chinese officials claim they were shut down over violations of fire safety regulations, but the timing is a bit suspicious, to say the least.

And, as my colleague Lindsay Maizland has written, “Korean TV shows and K-pop music videos have been blocked from streaming in China — one of their biggest and most lucrative markets — Chinese internet users have posted about boycotting Korean beauty products, and Korean celebrities have canceled tours in China.”

But fear of economic retaliation isn’t the only reason some South Koreans oppose THAAD. There are also safety and environmental concerns among local residents in the area where THAAD is being deployed.

Back in August, about 900 South Koreans shaved their heads in a mass demonstration against the government’s decision to house THAAD in the Southeastern county of Seongju, a region famed for its melon farming. Activists said they were concerned that the system’s sophisticated radar could harm their crops and that having a missile system nearby would potentially make the area a target in wartime.

At another demonstration in July, the governor of Seongju stood in front of a crowd of 5,000 protesters and wrote “No to the deployment of THAAD in Seongju” using his own blood. Literally.

The New York Times reports that some critics in South Korea are also upset with the government’s choice of Seongju as the THAAD site because putting it there will mean that the country’s capital, Seoul, will be outside the coverage of THAAD’s intercept missiles.

With the blessing of the previous administration in South Korea, the US quickly stepped up the timeline for THAAD’s deployment, in large part because of the increased belligerence from the North. And as of May 2, the system is operational.

But another reason for the rapid deployment was to make it that much harder for the next administration in South Korea to reverse the decision. Moon could still do so if he chooses, but it might be tricky.

“The next administration, however, will continue to face an excruciating dilemma,” writes South Korea expert Benjamin Lee in the Diplomat. “If South Korea decides to revoke the THAAD decision, this will set a terrible precedent, which will cause China to believe that it can use its economic influence over South Korea to control Seoul’s strategic agenda.”

Trump’s more aggressive approach to North Korea and the administration’s deployment of THAAD aren’t likely to be the only points of disagreement between the US and the incoming South Korean president. In fact, Trump’s aggressive approach to South Korea is almost certain to cause major tensions in the relationship between the two countries.

Yes, you read that right. As my colleague Zeeshan Aleem writes, “Apparently threatening to crackdown on one Korea wasn’t enough” for Trump.

In a wide-ranging interview with Reuters in late April, Trump made a number of stunning statements indicating that he plans to take a hard line with South Korea over trade and the cost of deploying THAAD.

The president said during the interview that he thought Washington’s free trade agreement with South Korea was such a bad deal for the US that he would withdraw from it if they were unable to renegotiate one to his liking.

“It’s unacceptable. It’s a horrible deal made by Hillary. It’s a horrible deal. And we’re going to renegotiate that deal, or terminate it,” he said. “It’s a great deal for South Korea. It’s a terrible deal for us.”

Trump also said that he expected South Korea to pay for THAAD.

“The THAAD system, it’s about a billion dollars. I said, ‘Why are we paying? Why are we paying a billion dollars? We’re protecting. Why are we paying a billion dollars?’” he said. “So I informed South Korea it would be appropriate if they paid.”

This, perhaps not surprisingly, did not go over particularly well in Seoul. However, shortly after that interview, NBC News reported that Trump’s national security adviser Gen. H.R. McMaster had quietly called the South Koreans to reassure them that the US would continue to pay for THAAD, directly contradicting his boss’s statement.

“The U.S. National Security Adviser H.R. McMaster explained that the recent statements by President Trump were made within the context in line with the general U.S. public expectations on burden-sharing with allies,” a South Korean presidential office spokesperson told NBC News.

This call may have somewhat assuaged the South Korean’s concerns over Trump’s rather hostile approach toward the longtime US ally, but the fact that the president made the comments in the first place will not easily be forgotten. How Moon will ultimately decide to handle THAAD — and Trump — remains to be seen. But one thing is for sure: a rift between the two longtime allies couldn’t come at a worse time.