Defense secretary was the last of a four-person group to balance the president’s more mercurial tendencies and analysts are alarmed

With Jim Mattis gone, has the last proverbial adult left the White House?

The departure of defense secretary Jim Mattis from the Trump administration, announced late Thursday, prompted concern about its timing, falling as it did one day after Donald Trump announced a surprise withdrawal from Syria and as reports circulated that the president was considering a similar withdrawal from Afghanistan.

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But anxieties were motivated by deeper concerns. For foreign policy analysts, government officials, allies abroad and observers at home, Mattis was the last of a four-person group of Trump administration heavyweights once seen as providing a corrective to the president’s more mercurial tendencies.

By this view, Mattis and his cohorts – national security adviser HR McMaster, chief of staff John Kelly and secretary of state Rex Tillerson – provided vital ballast for a ship of state prone to lurching in unpredictable ways if the president ordered, for example, the assassination of the Syrian president.

That precise order was given, journalist Bob Woodward recounted earlier this year in his book Fear. But the order was ignored by Mattis, a former commander of US Central Command known as “Mad Dog” from his time as a US Marines commander in the 2003 invasion of Iraq.

“We’re not going to do any of that,” Mattis reportedly told one of his aides. “We’re going to be much more measured.”

Now Mattis is gone, and the shaky question for many analysts is, where next?

This chaos, both foreign and domestic, is putting America in danger and must stop immediately John Kasich

“Never been more alarmed for the nation since coming to DC over three decades ago,” tweeted the conservative analyst and editor Bill Kristol.

“A former high-ranking Pentagon official told me recently that he could sleep at night despite Trump craziness – as long as Mattis was running the Defense Department,” tweeted Scott Stossel, an editor at the Atlantic. “Without Mattis, he worried, protection against bad decisions might have to be officers willing to disobey orders.”

McMaster stepped down in March, a week after Trump fired Tillerson, the former ExxonMobil chief executive who was reported to have called Trump a “fucking moron”, a report he did not deny. Kelly is to step down at year’s end. The president appears to be having trouble finding a permanent replacement as his chief of staff.

The stabilizing force of the foursome could be overstated. Neither McMaster nor Tillerson could manage Trump’s courtship of North Korea, friendship with Russia, attacks on European allies and obsessive slights against China.

Kelly appeared to stand foursquare behind Trump’s efforts to overstate a security threat at the southern border and impose unconstitutional restrictions on who might seek entry to the US.

None of the four prevented Trump’s withdrawal from key international initiatives such as the Paris climate agreement and the Iran nuclear deal.

But with Mattis gone, analysts exhibited newfound alarm, lamenting that the last proverbial adult in the room had left it.

“This chaos, both foreign and domestic, is putting America in danger and must stop immediately,” wrote John Kasich, the Ohio governor, on a day when Trump appeared to have precipitated a shutdown of the federal government shortly before the Mattis announcement.

Facebook Twitter Pinterest HR McMaster boards Air Force One with Donald Trump. Photograph: Mandel Ngan/AFP/Getty Images

“This is a sad day” tweeted Republican Nebraska senator Ben Sasse. “General Mattis was giving advice [Trump] needs to hear. Mattis rightly believes that Russia & China are adversaries, and that we are at war with jihadists across the globe who plot to kill Americans. Isolationism is a weak strategy that will harm Americans.”

Mattis explained in a resignation letter that his resignation was owing to differences with Trump on issues including how to treat allies and who counts as an enemy.

“My views on treating allies with respect and also being clear eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” the general wrote.

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That was a more benign criticism than Mattis has reportedly offered elsewhere. In a discussion of the North Korea issue, Woodward reported, it was Mattis’ appraisal that Trump “acted like – and had the understanding of – a ‘fifth or sixth grader’”. Mattis denied the quote.

Rumors that Mattis would leave had circulated for months. In September he denied them, telling reporters: “Of course I don’t think about leaving, I love it here.” But Trump said a month later that Mattis might leave soon.

“It could be that he is,” Trump said. “I think he’s sort of a Democrat, if you want to know the truth. But General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That’s Washington.”

On Thursday, one Washington figure who has clashed vehemently with Trump, Obama homeland security adviser and CIA director John Brennan, wrote on Twitter: “Okay, Republicans. How much longer are you going to let this farcical ‘presidency’ continue?

“At a time of such political, economic, and geo-strategic turbulence – both nationally and globally – are you waiting for a catastrophe to happen before acting? Disaster looms!”