SEOUL — Relationships, diplomatic or economic or personal, evolve. The U.S.-Korean relationship is not cast in stone. U.S. diplomats can go on all they want about the tightness of the bond, “no daylight between us” and all that, but it’s got to change with the times.

Probably no U.S. president has fortified that sense more clearly than Donald Trump. First, he just cannot get over his “love” for North Korea’s Kim Jong-un and his disdain for South Korea’s Moon Jae-in. That’s obvious from the way he praises Kim to the skies, including him with other dictators he admires like Russia’s Vladimir Putin and China’s Xi Jinping.

Trump may have trouble with some of their policies but thinks they’re wonderful. Sorry, he does not have similar feelings for Moon and is still holding up South Korea for considerably more than the $927 million paid last year for U.S. bases and troops. He has to know his original demand of $5 billion was stupid, but the U.S. would like a few hundred million more.

North Korea is hardly mentioned in a U.S. presidential campaign focused on domestic violence and COVID-19, but the North is sure to come up after the election.

Nobody would celebrate a Trump victory more than Kim Jong-un. He might order more missile tests, maybe a seventh nuclear test, if Biden wins but would hold off for another deal with Trump, who might well order more U.S. troops out of South Korea.

The vicissitudes of U.S.-Korean relations support the view of South Korea’s ambassador to the U.S., Lee Soo-hyuck, that the South doesn’t have to stick with its American alliance partner “just because South Korea chose the U.S. 70 years ago.”

That may seem a little controversial, but the South’s “special relationship” with Washington faces a cloudy future under either Trump or Biden. Whoever wins, “North Korea will look for something that indicates a new relationship with the U.S.,” said retired General Vincent Brooks, former commander of U.S. Forces Korea, in a zoom discussion in Washington.

And as “we get to inauguration” of the president in January, he went on, “North Korea will do something to test the newly inaugurated president.”

South Korea’s own policies are about as unpredictable as those of the Americans. Ambassador Lee has indicated the South might turn to China, which years ago replaced the U.S. as the South’s leading trading partner.

It’s beyond madness, however, to dream that China, as North Korea’s sole ally and benefactor, would stick up for the South. The Chinese can tell the North Koreans not to risk a second Korean War, but they’re not about to send troops and arms to defend South Korea.

Might Trump, Biden too, go along with Moon’s calls for a “peace declaration” formally ending the Korean War? “A declaration is far short of a peace treaty,” Brooks noted, but indeed could be “the starting gun of a new relationship.”

In any approach to a treaty, however, Kim would also demand withdrawal of all 28,500 U.S. troops now in South Korea while China still had plenty of armed might across the Yalu, or Amnok, and Tumen rivers from North Korea, ready to jump in on the North’s behalf.

Would Trump be dumb enough to fall for a treaty? Maybe, at least to judge from a tell-all book by his niece, Mary.

“Donald’s checkered personal history and his unique personality flaws make him extremely vulnerable to manipulation by smarter, more powerful men,” she writes in Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man. Tell him “he’s the smartest, the greatest, the best,” she goes on, and he’ll “do whatever they want….”

Kim has done all possible to flatter and cajole Trump since their first meeting in Singapore nearly two and a half years ago. Trump loves the way Kim addresses him as “excellency” in letters, according to Bob Woodward in his book Rage. Trump still has not withdrawn sanctions, one of the few non-lethal weapons the U.S. can deploy against the North, but he might do just that given the proper flowery words from Kim.

But then, how would Trump feel about China tightening its influence over both Koreas? That might be a tough one while Trump wages a boiling trade war with Beijing.

China’s not seriously getting the North to give up its nukes and missiles, and there’s not a lot either Trump or Biden can do without bringing armed pressure to bear that neither wants to try.