SEOUL — The president of Communist China, Xi Jinping,  has beaten the president of South Korea to a telephone conversation with President Trump.

Just as officials working for South Korea’s new president, Lee Jae-myung, were saying that Messrs. Lee and Trump would soon be talking, Mr. Trump chatted for an hour and a half with the president of the country that may be America’s foremost adversary. It was their first conversation since Mr. Trump’s inauguration in January.

Never mind that Mr. Trump’s defense secretary, Pete Hegseth, was warning a few days earlier, at a parley of many of the world’s top defense officials at Singapore, that the threat Communist China poses “is real, and it could be imminent,” and “any Chinese military move on Taiwan would result in devastating consequences.”

Such unpleasant stuff apparently was left unmentioned in what Mr. Trump described on Truth Social as “a very good phone call,” in which they discussed “some of the intricacies of our recently made, and agreed to, Trade Deal.” The talk, he wrote, “resulted in a very positive conclusion for both Countries.”

Trade issues are just as pressing for South Korea as for the People’s Republic of China. The Koreans would like to talk Mr. Trump out of the enormous tariffs that he’s imposed on steel, aluminum, motor vehicles, and other products. The previous conservative administration in Seoul asked Washington to stick to just 10 percent – far below the 50 percent hikes that Mr. Trump has talked about.

Mainland China, though, has a unique bargaining tool: rare earth. Mr. Trump after talking to Mr. Xi said, “There should no longer be any questions respecting the complexity of Rare Earth products.” Negotiators, he said, will “work out a deal.”

Unlike China, Seoul also has much to discuss about its defense relationship with Washington. Mr. Trump may want to talk about pulling some of America’s 28,500 troops from Korea. Too, he may ask Seoul to contribute more than the current $1.1 billion a year toward the costs of hosting American troops and bases. That includes America’s largest overseas base at Camp Humphreys, 40 miles south of Seoul.

Mr. Lee, who rose to power as a leftist, has often cited the need for balancing Korea’s relations between Washington and Beijing. A familiar refrain runs: “We should not put all our eggs in one basket.” He has also called for reopening dialog with North Korea’s leader, Kim Jong-un — a goal that may appeal to Mr. Trump, who met Mr. Kim three times in 2018 and 2019. 

So, South Koreans are asking, what are they waiting for? Why aren’t Messrs. Trump and Lee talking? Mr. Trump’s “silence on Lee Jae-myung raises eyebrows in Seoul and Washington,” a headline over an article on NK News, a website in Seoul, reads. 

The two presidents “have yet to hold a phone call, breaking with the longstanding custom in which newly inaugurated leaders from both countries typically speak within hours of taking office,” the article says. “Citing scheduling constraints and time differences, South Korean presidential officials told local media that the two sides are still ‘coordinating’ the call’s timing but did not offer a target date.”

Adding to the mystery is that Mr. Trump has not sent a message personally congratulating Mr. Lee on his victory over the conservative, Kim Moon-soo, in Tuesday’s election. Mr. Lee “has received congratulatory messages” from Mr. Xi and the Japanese prime minister, Shigeru Ishiba. Secretary Rubio has also sent a congratulatory message.

One hang-up may be Seoul’s reluctance to get involved in the defense of the island province of Taiwan that Mr. Xi claims as part of China. That topic did not come up in Mr. Trump’s conversation with Mr. Xi, but it may be a sticking point between Washington and Seoul.

Mr. Lee says “closer relations with China will occur within the framework of South Korea’s alliance with the US,” a professor of international relations at England’s University of Bradford, Christopher Bluth, writes. “But, with Washington and Beijing battling for global influence, this is still likely to become a major point of tension with the US. The Trump administration has taken a hawkish approach towards China and wants its allies to do the same.”