During his 2024 election campaign, Donald J. Trump described the United States’ alliance with South Korea as a terrible bargain for his country, accusing the Asian ally of not paying enough ​for the 28,500 ​American troops ​stationed on its soil.

But when he mentioned Kim Jong-un, the leader of North Korea who has threatened to use nuclear weapons against the South, he talked as if Mr. Kim were a ​long-lost friend.

“It’s nice to get along when somebody has a lot of nuclear weapons or otherwise,” Mr. Trump said about Mr. Kim in July. “He’d like to see me back, too​. I think he misses me​, if you want to know the truth.”

South Koreans are bracing themselves for ​Mr. Trump’s return to the White House with growing anxiety and uncertainty, as it raises the specter of the diplomatic roller coaster ride on the Korean Peninsula they endured during his first term.

Some fear that Mr. Trump will once again threaten to withdraw American troops from South Korea unless it greatly increases its share of the costs, and that he will rekindle an ill-calculated diplomatic bromance with Mr. Kim​.

“South Korea-U.S. relations will sail into a storm,”​ said Lee Byong-chul, a North Korea expert at the Institute for Far Eastern Studies in Seoul. “But we will likely see Kim Jong-un and Trump exchanging love letters again.”