James Mattis’s departure represents a serious diminution in the experience and wisdom in the White House

He was one of the last ‘grown-ups in the room’, to use the well-worn expression that the president apparently found infuriating

Another one bites the dust. The resignation of James Mattis, the US defence secretary, is, it is worthy reminding ourselves, only the latest in a bewildering volume of personnel changes in and around the Trump administration – probably the most unstable US government since the civil war.

Mr Mattis joins Jeff Sessions, HR McMaster, Rex Tillerson, Gary Cohn, Omarosa Manigault Newman, Steve Bannon, Reince Priebus, Sean Spicer, Anthony Scaramucci... Some served longer than others, and some were certainly a greater loss to public life than others, but the very mention of some of these names can still make you wince.

Lest there be any doubt, Mr Mattis’s departure represents a genuine and serious diminution in the experience and wisdom around the cabinet table – he was one of the last “grown-ups in the room”, to use the well-worn expression that the president apparently found infuriating.

Mr Mattis has resigned for honourable reasons. He put it politely in his resignation letter, and honestly – he and the president differ on important issues of policy, and it would obviously be preferable to have a defence secretary that is in complete harmony with the commander-in-chief.

The decisions to withdraw US troops entirely from Syria, and to withdraw half the presence in Afghanistan, seem to be have been the last straws for Mr Mattis. By nature cautious and conservative – we are talking here about a four-star general and the security of the United States and of the west, after all – he clearly judged that the risks of pulling out now were greater than any possible military or other advantage.

In particular, America’s long disengagement from Afghanistan invites the possibility that this poor, fractured state will revert to its previous status as a playground of international terrorists – the people who engineered the 9/11 attacks, among others. It seems to have been too easily forgotten how the Taliban were able to host al-Qaeda, and how the effort to eliminate Osama bin Laden in Afghanistan was an international one, backed by the United Nations and Nato. America had been attacked; its allies, and the wider world, rushed to its aid. It is ironic that America should now be so keen to risk the security of the whole world by permitting Isis to pick up where the Taliban and al-Qaeda left off.

Withdrawing from Syria, too, invites the Russians and the Assad government to re-establish control over the country – but it is unlikely that their brutal methods, including the use of chemical weaponry on civilians, will deliver peace. The Trump administration’s reckless sponsorship of the Netanyahu government – symbolised by the decision to switch the US embassy from Tel Aviv to Jerusalem; the reimposition of tough sanctions on Iran; the signalling of US weakness in Syria and Afghanistan: it all means greater violence across a huge swathe of Asia, from the Mediterranean coasts of Lebanon and Israel to Pakistan’s border with India.

And yet no one should say they are surprised by what has happened, though the extent of the chaos inside the White House can be bewildering. When Americans voted for Mr Trump in 2016, they knew more than enough about his brittle, volatile temperament. They were well aware too that he wanted to bring US forces home and end the country’s entanglements in faraway lands.

For that matter, Americans were also well aware he was prone to start a trade war with China, prepared to build a ridiculous wall at the Mexican border, and had a complicated relationship with Vladimir Putin. Many will be terrified by the way Mr Trump is running this superpower; but many of his “base” will be unconcerned about the departure of yet another senior public official, as they’ve not cared much about any of the others. All they care about is that the Donald keeps his promises. Depressingly, and no matter what the damage, he seems determined to do just that.