White House chief of staff John Kelly persuaded U.S. President Donald Trump not to order the withdrawal of all American troops from the Korean Peninsula ahead of the February Winter Olympics in South Korea, a report said Monday, just weeks ahead of a planned summit with North Korean leader Kim Jong Un.

The report by NBC News, citing two unidentified administration officials, said that “in one heated exchange between the two men … Kelly strongly — and successfully — dissuaded Trump from ordering the withdrawal of all U.S. troops from the Korean Peninsula.”

The U.S. stations 28,500 troops in South Korea as a legacy of the Korean War, which ended in an armistice, not a peace treaty. Any decision to withdraw U.S. forces from the South would have wide-ranging effects, including on Japan’s security calculus, according to experts.

In a statement released later Monday, Kelly blasted the report, in which he was also said to have privately disparaged Trump as “an idiot” last year, calling it “total BS.” He framed his relationship with Trump as “incredibly candid and strong.”

NBC said three White House spokespeople had played down the apparent push to remove American troops from South Korea, saying “they have not heard Trump talk seriously” about the idea.

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The NBC report, if true, comes after a stunning revelation by U.S. Secretary of Defense Jim Mattis last Friday that the issue of keeping troops on the peninsula would be discussed with not only allies, but also North Korea — a prospect unthinkable just months ago.

“I think for right now we just have to go along with the process, have the negotiations and not try to make preconditions or presumptions about how it’s going to go,” Mattis said.

The North, which had long demanded that U.S. troops be withdrawn — citing their presence as an invasion threat and a pretext for its nuclear weapons program — has said that Kim would not ask for their removal as a precondition for abandoning his nuclear weapons, South Korean President Moon Jae-in said last month.

Trump is due to meet Kim in “three or four weeks,” the U.S. president said Saturday.

That meeting would come after the pageantry of last week’s third inter-Korean summit, the first in more than a decade.

Kim met the Moon on the southern side of the Demilitarized Zone, where the two agreed to achieve “a nuclear-free Korean Peninsula through complete denuclearization,” rather than clearly stating “a nuclear-free North Korea.”

The North has long said the term “denuclearization of the Korean Peninsula” must include the United States withdrawing its troops from South Korea and removing its so-called nuclear umbrella security commitment to South Korea and Japan.

The NBC report said that Kelly, who had been tapped as chief of staff to instill order in a chaotic White House, had pushed back against Trump on a limited number of foreign policy and military issues, according to former and current officials.

It said the exchange about troops in South Korea had underscored the rationale behind one of Kelly’s more common refrains, which multiple officials described as some version of “I’m the one saving the country.”

“The strong implication being ‘if I weren’t here we would’ve entered WWIII or the president would have been impeached,’ ” the report quoted one former senior White House official as explaining Kelly’s stance.

Vipin Narang, a North Korea expert and professor of international relations at MIT, said that any move by Trump to withdraw American troops from the peninsula could “arise from his belief that the U.S. was not being compensated appropriately by South Korea for the U.S. military presence there — both directly and indirectly in trade — rather than questioning the security rationale for U.S. extended deterrence commitments.”

For allies like Japan and South Korea, this rationale could be an ominous portent.

Trump has at times linked U.S. military commitments in both nations to trade, while also arguing that the allies need to cough up more cash for the American troop presence.

“We defend” Japan and South Korea … but “they pay us a fraction of what it costs,” he said in February. “We’re talking to all of those countries about that because it’s not fair that we defend them.”

During the 2016 U.S. presidential election campaign, Trump repeatedly bashed Japan for not covering enough of the costs of stationing American troops in the country.

Narang said that Trump “may not care” about or “understand” the security issues, and that policies “that were taken for granted by the U.S. allies are perhaps no longer sacrosanct.”

“If Trump’s instincts continue to favor a reduced footprint for economic reasons, and if Moon and Kim propose it, any reconfiguration could be possible,” Narang said of a U.S. troop withdrawal. “And yes, allies from Japan to NATO might be rightfully concerned that they might be on the chopping block next.”