Can South Korea Manage the Competing Needs of the U.S. and China?

China’s leader, Xi Jinping, on Saturday called on South Korea to deepen cooperation with Beijing as he met with the country’s president, Lee Jae Myung, during his first visit to the country in 11 years.

But South Korea, like other Asian nations that have strong ties to China and the United States, has found it increasingly difficult to balance relations between them as the rivalry between Washington and Beijing has intensified.

Mr. Lee took office in June, vowing to improve ties with China. Mr. Xi’s visit this week to attend the Asia-Pacific Economic Cooperation meeting in the city of Gyeongju was originally envisioned as a chance to do so through more economic collaboration.

But those efforts were complicated by South Korea’s attempts to also strengthen its longstanding alliance with the United States under President Trump, who also attended the APEC summit and held talks with Mr. Lee on Wednesday.

This week, Mr. Trump created a new potential flashpoint by announcing that the United States would allow South Korea to build nuclear-powered submarines, a move that will further integrate the country into Washington’s security architecture.​ The South Korean leader had lobbied for Mr. Trump’s backing, saying the submarines would help the country’s navy to​ patrol waters near China and North Korea and lessen “the burden” on the U.S. military. But the president’s sudden announcement caught even South Korean officials by surprise.

On Saturday, Mr. Xi said China and South Korea were inseparable partners and were prepared to deepen ties. But he also urged South Korea to reject American-led efforts to suppress trade with Beijing and “uphold the multilateral trading system.”

Mr. Lee emphasized the importance of trade ties with China, which he said had helped South Korea become a global economic powerhouse.

Despite the encouraging words, however, the APEC summit this week showed how tricky it will be for South Korea to manage the competing needs.

“South Korea has long balanced its security dependence on the United States with its economic interdependence with China,” said Seong-Hyon Lee, a senior fellow at the George H. W. Bush Foundation for U.S.–China Relations. “That equilibrium has effectively ended.”

Yet even as the United States has urged South Korea to join U.S.-led efforts to contain China, Seoul has also sought Beijing’s help to manage tensions with North Korea. Mr. Lee said this week that Beijing, an ally of Pyongyang, would have “a big role” in building peace on the Korean Peninsula.

​“Stability on the Korean Peninsula and in Northeast Asia benefits China too,” Mr. Lee said during a news conference Saturday at the end of the APEC summit.

The deep divide between the United States and China cast a shadow over the summit despite Mr. Trump and Mr. Xi agreeing a trade truce this week.

On Wednesday, Mr. Trump extolled his “America First” agenda and use of tariffs as part of his foreign policy in a speech to business leaders in Gyeongju.

Two days later, at the APEC leaders’ summit, Mr. Xi warned nations against joining the United States in ​reducing the world’s reliance on Chinese supply chains.

Mr. Lee said that APEC member states haggled until the last minute over the joint statement at the end of the summit, reflecting how difficult it has become to find common ground.

“In the current environment of norm-violating tariffs, export controls, and industrial policies, it is difficult to build consensus on international institutional reform and free trade efforts,” said Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul.

He promised Mr. Trump that South Korea would increase military spending, something the president has encouraged to help counter China. Their meeting ended in a high note for Mr. Lee, resulting in a trade deal and the agreement on nuclear-powered submarines that Mr. Trump said on social media would be built in Philadelphia.

South Korea has also agreed to invest $350 billion in the United States, including in its shipbuilding industry, in exchange for the Trump administration agreeing to lower tariffs on its goods to 15 percent from the 25 percent levy it imposed in August.

South Korea gave a ​similarly warm reception to Mr. Xi. A traditional Korean honor guard escorted the ca​r transporting the Chinese leader as it pulled into the Gyeongju National Museum​, the venue for the summit.​

The efforts to improve ties follow a decade of difficult relations.

In 2014, ​Mr. Xi visited South Korea, five years before making his first trip to North Korea. Until then, a new Chinese leader had always visited the North first. The then South Korean leader, Park Geun-hye, reciprocated the following year by standing next to Mr. Xi as they watched a military parade in Beijing to mark the 70th anniversary of the end of World War II.

But when China held a similar military parade in September to mark the 80th anniversary of the war’s end, the South Korean leader’s spot was taken by Kim Jong-un, North Korea’s leader — a sign of how relations between Seoul and Beijing have soured over the last decade after South Korea decided to let the United States deploy an antimissile system on its soil.

Since Mr. Trump was re-elected, South Korea has taken steps to ameliorate the president’s threats in ways that could upset Beijing again.

The submarine agreement, coupled with South Korea’s massive investment pledge in the U.S., “signals a deep, structural integration of South Korea into America’s security and industrial ecosystem,” said ​Mr. Lee of the George H. W. Bush Foundation​.

China has already expressed its discomfort. On Friday, Guo Jiakun, a spokesman for China’s foreign ministry, said South Korea and the United States should “do what is conducive to regional peace and stability, not otherwise.”

David Pierson contributed reporting from Gyeongju.