Although former US Defence Secretary James Mattis, after the release of his memoirs — “Call Sign Chaos: Learning to Lead” — has said he will “not speak ill” of a sitting US president, he clearly found Donald Trump’s decision to withdraw troops from Syria a grave mistake.

Indeed, he said Trump’s precipitous decision in December 2018 to pull out of Syria was the main reason for Mattis’s resignation. He had planned to be the Pentagon chief for the full four years of the administration’s first term but, out of principle, said he could no longer serve a president whose views were in sharp contrast to his.

In an interview with CBS television, Mattis emphasised: “We need to maintain enough influence there [Syria] that we don’t see the same thing that happened when we withdrew from Iraq.” Mattis obviously believed that Trump’s eagerness to declare victory over the Islamic State (ISIS) was premature.

Mattis also underscored the importance of maintaining alliances: “This is how I saw the strength of America — that we keep our alliances together and keep them tight.”

In this case it seems that Mattis was referring to the alliance the United States made with the Syrian Kurds, who fought hard and suffered many casualties in the anti-ISIS campaign.

The implication of his statements was that, by withdrawing from Syria, the United States would not be in position to influence the future of Syria nor help its European and Middle Eastern allies stem a possible resurgence of ISIS. He also implied that withdrawing from Syria would expose the Kurds to danger because they would no longer be under a US protective shield.

On the issue of an ISIS resurgence, the views of Mattis have been borne out by public assessments of the US intelligence community as well as by UN experts who say that at least 15,000 ISIS fighters are at large and operating in cells in Syria and Iraq. Indeed, they have staged terrorist attacks in both countries in recent months.

Even before the Syrian withdrawal decision, Mattis’s star was falling in the eyes of Trump. Mattis, along with former US Secretary of State Rex Tillerson, had reportedly urged Trump not to withdraw from the Iran nuclear deal, which Trump did in May 2018.

Even though Mattis had strong anti-Iran views — he was sacked as US CENTCOM commander by President Barack Obama for advocating a military strike on Iran, his memoirs state — he believed it was wrong to back out of a deal done in conjunction with European allies.

Mattis disagreed with Trump’s often disparaging views on NATO, a cornerstone of US security since the late 1940s and Trump’s indulgence of Russian President Vladimir Putin and North Korean leader Kim Jong-un.

In October 2018, Trump told an interviewer that he did not know whether Mattis would be leaving his administration. Trump then referred to Mattis as “sort of a Democrat” and claimed he knew more about NATO than his defence secretary. Rumours circulated in Washington at the time that Trump was upset that journalists referred to Mattis as the “adult in the room” when in the White House, implying that Trump was acting like a child.

Although Trump was initially enamoured with Mattis’s distinguished military record, by the autumn of 2018 he had clearly soured on him. When Mattis submitted his resignation letter in December, Trump did not even allow him the courtesy of remaining in the job for two more months as he wished but replaced him almost immediately.

Some defence specialists in Washington criticised Mattis for not being more forceful in his memoirs in denouncing Trump — the most Mattis has said in interviews is that Trump is an “unusual president.” Having taken the step of writing his memoirs, which include his time in the Trump administration, Mattis should have been more direct and say where the United States should be headed, the specialists say.

Mattis has said he did not want his memoirs or subsequent interviews to contribute to the toxic political environment in Washington, which he sees as dangerous.

Nonetheless, Mattis is held in high regard among the Washington political class, Republicans and Democrats alike. His warnings on Syria and ISIS are likely to be picked up by Democratic presidential candidates who will use them to disparage Trump as both naive about ISIS and the Middle East region as well as a sign of his erratic behaviour.

The irony is that Trump did amend his Syrian withdrawal decision by agreeing in 2019 to keep about 400 US troops in Syria. That is of small consolation to Mattis and his supporters who see the US president as making policy on the whim without serious deliberations.