The resignation of US Defense Secretary James Mattis, announced by US President Donald Trump last week, has increased concerns about growing uncertainty over the future of the South Korea-US alliance and Washington’s approach to North Korea.

Retired four-star Gen. Mattis is considered to have kept Trump in check, preventing the president from making impulsive decisions that might have pushed the Korean Peninsula to the verge of a full-scale military conflict or unraveled the bilateral alliance between Seoul and Washington.

He reportedly held the US leader back from resorting to military options to revolve the nuclear standoff with the North or pulling American troops from the South.

His resignation comes at a time when President Moon Jae-in’s government in Seoul seems to be out of tune with the Trump administration over steps to be taken over Pyongyang, which appears to be changing its definition of denuclearization.

Trump announced Thursday that Mattis would be retiring “with distinction” at the end of February, a day after the US president’s controversial declaration that all US troops would be withdrawn from Syria. Mattis had warned that an early pullout from the country would be a strategic mistake.

In his resignation letter, Mattis reiterated his support for a US network of alliances, expressing his views on “treating allies with respect” and “using all the tools of American power to provide for the common defense.”

“Because you have the right to have a secretary of defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down,” he said.

Trump’s abrupt decision to withdraw the 2,000 US troops from Syria -- likely to be followed by an announcement of a partial pullout from Afghanistan -- was, as put by a Republican senator, a transactional and political decision not based on facts on the ground.

It was apparently directed at his supporters at home, ignoring the possibly disastrous fallout on Kurdish-led militias at the forefront of the battle against Islamic extremists in northeast Syria.

The move might embolden North Korea to demand that US troops leave the South before an agreement on dismantling its nuclear arsenal is reached.

Trump himself might be willing to withdraw at least part of the about 28,000 US soldiers stationed in South Korea, in return for initial progress in the process of Pyongyang’s denuclearization.

There is a possibility that he might seek the option if Seoul’s concessions in ongoing talks on sharing the cost of stationing US forces here fall short of his expectations. The two sides have yet to reach an accord after holding 10 rounds of negotiations, with Washington calling for a 50 percent increase in the financial burden shouldered by Seoul, which was set at 960.2 billion won ($853.8 million) this year. Trump had previously hoped that Seoul would double its financial burden and cover the full cost of keeping American troops on South Korean soil.

Under a law that takes effect next year, Trump will be prohibited from reducing the number of US soldiers here to below 22,000 without congressional approval. This means that he would be free to cut the troop size by about 6,000 if he decides to do so.

A greater cause for concern is that Trump might go back to military options if talks with the North continue to stall. The Trump administration has kept the door for negotiations with Pyongyang open. But Pyongyang has recently shown signs of going back on its commitment to denuclearization.

In a dispatch last week, North Korea’s state news agency insisted the denuclearization of the Korean Peninsula referred to the elimination of nuclear threats from the US ahead of scrapping “our nuclear deterrence.”

Mattis’ successor, whose views would be better aligned with Trump’s, can hardly be expected to provide the US leader with the kind of sound and balanced advice that the outgoing defense secretary has tried to get through to the Oval Office.

With the last remaining grown-up in the chaotic Trump administration leaving the scene, it is more necessary for the Moon government to ditch its wishful thinking in denuclearizing the North and handling the alliance with Washington. It should make preparations for the worst scenario and stop what critics say is a naive and unrealistic diplomatic push reliant solely on the goodwill of its counterparts.