SEOUL, South Korea — Lee Jae-myung, sworn in on Wednesday as South Korea’s president, faces an immediate problem with the tariffs President Trump has threatened to put into effect next month.

If the career politician can defuse that crisis, a bigger one is waiting in the wings. It’s a complicated bilateral reevaluation of U.S. forces’ role in defending South Korea against its longtime antagonist, North Korea, or an increasingly aggressive China.

The Trump administration wants South Koreans to pay more for U.S. protection. Mr. Lee and other prominent South Korean leaders are openly questioning whether it’s time to reduce the country’s dependence on the U.S.

“We will strengthen South Korea-U.S.-Japan cooperation based on a solid Korea-U.S. alliance,” said Mr. Lee, who affirmed the importance of the U.S. relationship in his first speech as president while signaling his country was at a crossroads.

South Korea will “approach relations with neighboring countries from the perspective of practicality and national interest,” he said.

That may raise eyebrows in regional democracies and Washington, where the Trump administration seeks to end conflicts in Europe and the Middle East to free up U.S. resources to concentrate on China and the Indo-Pacific.

As Mr. Lee, 61, takes office, reports and rumors in Washington and Seoul raise questions about the future of the 28,000-strong U.S. Forces Korea.

One question is whether Mr. Lee and Mr. Trump can cut a deal on “cost sharing” to pay for U.S. troops on the Korean Peninsula. On April 8, Mr. Trump wrote on Truth Social that he had discussed and was making progress on “payment for the big time Military Protection we provide to South Korea” with Mr. Lee’s predecessor.

Mr. Lee has indicated openness to an agreement tying tariffs and cost sharing, as Mr. Trump has suggested.

Those negotiations will almost surely include discussions on whether Mr. Trump will downsize the number of U.S. troops in South Korea, perhaps by as many as 4,000 soldiers. More crucially, the two sides will discuss whether the American forces will defend South Korea not just against the North Koreans but also against the worst-case scenario of an attack by China.

Mr. Lee needs solutions to several issues, but a South Korean expert who has studied the alliance said the U.S. is really asking just one question of the new South Korean regime.

“Reduction of forces, burden sharing, force flexibility — these are all the same question from the U.S.,” said Yang Uk, a security analyst at Seoul think tank the Asan Institute for Policy Studies. “It asks, ‘Are you really our ally?’”

Shifting security sands

Per a 1953 security arrangement, U.S. forces remain deployed on the peninsula to deter North Korea, while Seoul and Washington are bound by a mutual defense treaty.

Now, matters are more complex.

China’s rise to the world’s No. 2 economic position makes it a key trade partner for South Korea. Meanwhile, Beijing’s expansionist pressures in the region pit it against regional democracies.

These factors have Seoul policymakers on a tightrope, doubly so as senior U.S. officials talk less about North Korea and more about China.

Last month, the top U.S. commander in South Korea, Gen. Xavier Brunson, discussed South Korea’s geopolitical position during a speech at the U.S. Army’s Land Forces Pacific Symposium in Hawaii.

“Korea is on the Asian continent, has a sizable U.S. force posture, is inside the First Island Chain and is the closest allied presence to Beijing,” he said. “At night, from a satellite image, [South Korea] looks like an island or fixed aircraft carrier floating in the water between Japan and mainland China.”

“This is a really important piece of real estate for U.S. interests,” said Chun In-bum, a retired South Korean general. “A lot of Koreans don’t agree with me; they need to change their minds, pronto.”

A traditional proverb calls Korea a “shrimp between whales,” a small kingdom crushed between larger powers. The cliche retains millennial resonance.

In 2017, U.S. forces deployed a THAAD missile defense battery in South Korea, complete with powerful radar that can scan the atmosphere of northeast China’s. Beijing retaliated with economic pressure, but not against the U.S. South Korean businesses suffered the fallout.

America’s forward operating base?

Gen. Brunson made clear that U.S. forces in South Korea are not postured solely against North Korea, as many South Koreans assume.

“Our presence in Korea imposes costs and changes the calculations and decisions of leaders of [North Korea], Russia and China,” he said. “We can … lever our geography and positional advantage to great effect.”

Although Chinese and South Korean claims in the Yellow Sea have some overlap, Chinese forces are not actively operating against South Korean forces. They are against Japan, the Philippines and Taiwan.

However, South Korea may lose its immunity to regional crises.

“Take a look at a map,” Gen. Brunson said. “Draw a line from Korea to Japan to the Philippines. …  What you find there is a triangle of nations inextricably linked by mutual defense treaties to the U.S. and nations who will undoubtedly be impacted by any crisis or conflict in the Taiwan Strait.”

The changing U.S. boot print on the peninsula backs Gen. Brunson’s words.

The United States Forces Korea, no longer positioned in the Demilitarized Zone, are heavily concentrated in a string of bases south of Seoul on Korea’s east coast facing China across the Yellow Sea.

Forty miles south of Seoul, the biggest U.S. overseas base is Camp Humphreys, headquarters of the U.S. 8th Army, near the seaport city of Pyeongtaek.

North of Pyeongtaek is the 7th Air Force base at Osan, which includes a Space Force command element. South of Pyeongtaek, in Kunsan, is K-8 Air Base, which has more U.S. aerial assets.

Although the U.S. naval presence is lacking, South Korea’s naval base at Pyeongtaek frequently hosts U.S. warships, as does another South Korean naval base on Jeju Island at the entrance to the Yellow Sea.

A Chinese fleet command, naval bases and naval shipyards are set around the Yellow Sea.

The U.S. Army’s website calls Camp Humphreys “the largest power projection platform in the Pacific.”

“Korea is a really, really good place to monitor and counter China,” Mr. Yang said.

Statements like that do not sit well with Wi Sung-lac, a South Korean lawmaker appointed Wednesday as Mr. Lee’s national security adviser.

“I understand what you mean by force projection,” he told The Washington Times last month. “I don’t believe these bases were designed to deal with the China problem. … We believe, still, that the main purpose of USFK and bases in Korea are to deal with the threat from the North.”

Maintaining defenses against North Korea is a strong argument against using USFK forces off the peninsula. There are also legal issues.

U.S. bases are “on the sovereign territory of South Korea,” said Dan Pinkston, an international relations professor at Seoul’s Yonsei University. He recalled that some European nations refused U.S. forces overflight permissions during the Persian Gulf War.

“U.S. assets could fly out, but South Korea could deny them landing rights coming back,” he said.

That would have ramifications.

“There are … tangible costs of not supporting an international defense of Taiwan,” said Bruce Klingner of The Heritage Foundation. “South Korea exists today because 70 years ago an international coalition defended it against attack and guaranteed its sovereignty.”

Seoul’s reluctance to defend a fellow democracy “could have significant repercussions for its alliance with the United States,” he said.

If GIs fight a regional conflict, USFK must engage, Gen. Chun said.

“Though USFK’s main mission is absolutely the defense of Korea and the stability of Northeast Asia, if the U.S. is in trouble in another Indo-Pacific area, it is only natural that it would send troops and request its allies for help,” he said. “That’s common sense.”

That makes South Korea a potential Chinese target, a possibility that goes largely unspoken.

“If conflict happens between the U.S. and China, any USFK installation would naturally be a target for China … We’d be drawn into war,” said Mr. Yang. “The risk is there, but Korean politicians and generals don’t want to accept it in public.”

• Andrew Salmon can be reached at asalmon@washingtontimes.com.