South Korea’s new president, Lee Jae-myung, has upset historic Korean-American ties by pursuing what he calls a “pragmatic” policy of attempting to play Washington against Beijing.

Washington has signaled its discontent by abruptly canceling a visit to Seoul by Secretary Rubio. That came after Mr. Lee invited Communist China’s party boss, Xi Jinping, to South Korea for an  October meeting of leaders of member states of the Asia-Pacific Economic Cooperation group.

Mr. Lee is also expected to go to Beijing to attend a parade marking the 80th anniversary on September 3 of what Beijing celebrates as China’s “victory” over Japan, which had already surrendered to the Americans on August 15, 1945. Mr. Rubio’s decision “has fueled concerns in Seoul that Washington’s shifting diplomatic priorities could undermine coordination on North Korea policy,” Seoul’s NK News reported.

The stated reason for the cancellation was that Mr. Rubio had to be on hand Monday as President Trump hosts Prime Minister Netanyahu at the White House for their first meeting since the “12-day war” that ended when American B-2 bombers blasted Iranian nuclear and missile facilities with bunker-buster bombs after the Israelis had bombed and shelled them.

Mr. Rubio, however, did not state that he was postponing the meeting in anticipation of a later visit. Instead, he put off what were expected to be crucial talks on America’s military commitment to South Korea’s defense. America has 28,500 troops in the South. Mr. Trump has hinted that he might want to withdraw several thousand of them while asking Seoul to contribute far more than the current $1.1 billion a year as its share of the costs.

South Korea’s national security adviser, Wi Sung-lac, is to spend three days at Washington this week focusing on another sensitive area in American-Korean relations — the 25 percent tariffs that Washington is about to impose on imports from South Korea, including motor vehicles. A former Korean ambassador to Moscow, Mr. Wi said Seoul and Washington were “closely discussing trade and security issues” at “a critical phase,” according to Seoul’s Yonhap News.

“Under normal circumstances, South Korea would be well placed to compete in the U.S. market” on the basis of the existing free trade agreement, a former director of the Hyundai Motor-Korea Foundation Center for Korean History and Public Policy, Tony Stangarone, writes. Now, however, “South Korea will need to reach a new understanding on trade to minimize the damage to key sectors such as automobiles and semiconductors.”

On a larger scale, the cancellation of Mr. Rubio’s visit to Seoul “has fueled concerns over the lack of high-level exchanges between the allies,” Yonhap reports. Basically, Washington and Seoul have had an uneasy relationship since the crisis that began with the declaration of martial law in December by the man who was then South Korea’s president, Yoon Suk-yeol.

Impeached and ousted, the conservative Mr. Yoon is now on trial for “insurrection” while his long-time nemesis, Mr. Lee, the left-leaning leader of the Minju, or Democratic, party, looks for talks with North Korea’s leader, Kim Jong-un. Mr. Wi sought to soften the impact of Mr. Lee’s invitation to Mr. Xi by saying Mr. Lee also hoped to meet President Trump. He may even hint at such an invitation if he gets to see Mr. Rubio.