With James Mattis gone, the last of the grown-ups has left the Trump administration

The defence secretary's resignation after the president's reckless decision to withdraw US troops from Syria is to his credit, but the world's detriment

In his resignation letter, James Mattis outlined clear ideological differences between himself and Donald Trump. Bloomberg

The resignation of defence secretary James Mattis last week bears the hallmarks of a significant turning point for the Trump administration. What it stands for in foreign policy, and how it is likely to conduct itself for the second half of its term are now much clearer – and the implications are alarming.

Mr Mattis cited a broad philosophical disagreement with President Donald Trump. In his resignation letter, he wrote of “treating allies with respect”, “being clear-eyed about both malign actors and strategic competitors”, and doing “everything possible to advance an international order” as core principles to which he is committed. His letter left no doubt that Mr Trump does not share these commitments.

The Trump administration has been steadily shedding the “grown-ups” – seasoned and accomplished professionals and practitioners, such as Rex Tillerson, HR McMaster, Gary Cohn and John Kelly – and now no one with the will, credibility and gravitas to challenge or restrain Mr Trump is left.

Worse, "America first" – the amorphous phrase Mr Trump uses to describe his policies – is finally starting to become clearly defined, in a disturbing manner.

The Trump administration has thus far been Janus-faced, especially in foreign policy.

At times, it has pursued a kind of robust internationalism recognisable from Republican Cold War traditions. But at other times it has appeared to be neo-isolationist and, at best, mercantilist, by emphasising the expansion of exports at the expense of all other goals.

Mr Trump's idiosyncratic views – such as hostility to trade, multilateral alliances, and traditional partners, his commitment to bilateralism in all things, and his strange affinity for dictators rather than the elected leaders of other democracies – were often held in check during his first two years in office by the dwindling cadre of “grown-ups” around him.

Now that they are all gone, there is no one left who is either willing or able to tell the president when he’s about to make a colossal blunder.

The proximate cause of Mr Mattis's resignation – Syria – is a perfect case in point.

Mr Trump has long wanted to pull American troops out of Syria and Afghanistan. He has said so many times, and he has tried to order the withdrawal of US forces from Syria in the past.

But Mr Mattis and others repeatedly confronted him with the implications of such a reckless action, and each time he capitulated.

Apparently, he’s no longer willing to do that.

It is widely reported that a week ago Mr Trump called Turkish President Recep Tayyip Erdogan to warn him not to attack US-allied Kurdish forces in northern Syria. His staff had created a well-thought-out agenda and carefully constructed talking points to try to achieve that uncontroversial goal.

Then Mr Trump went wildly off script.

When Mr Erdogan urged him to withdraw US forces from Syria, Mr Trump shocked both his own staff and the Turkish leader by readily agreeing, leaving Mr Erdogan as the one suddenly warning about the dangers of a precipitous US exit.

But it was too late. Mr Trump was determined to follow through with this folly, no matter what.

Almost no one supported his decision among his own senior staff or knowledgeable Republicans in Congress. Many pointed out that this would be a huge victory for the Assad regime, Vladimir Putin and, above all, Iran, and noted that ISIS is by no means defeated and could well stage a major comeback in the vacuum left by US forces.

They also pointed out that this was an unconscionable betrayal of the Kurdish and Arab fighters who have been the main US ground forces against ISIS, and are now being offered up to their Turkish and Syrian regime enemies.

Mr Mattis went to see Mr Trump in a last effort to convince him to avoid this historic strategic, political and moral misstep.

He was unsuccessful, and therefore resigned.

The lessons are unmistakable and deeply alarming.

Following this episode, and with no “grown-ups” left, there is no one to restrain Mr Trump's worst impulses. And "America first" now plainly involves some mixture of irresponsible neo-isolationism and an almost wilful disregard for the interests of allies.

It’s understandable that Mr Mattis can no longer, in good conscience, be part of this fiasco.

Major foreign-policy decisions made on the basis of delusions (that ISIS has been thoroughly defeated), disregard of consequences (huge victories for Iran and ISIS), historical amnesia (how ISIS emerged from a similar rushed withdrawal from Iraq), and against the advice of all experts, officials and, especially, allies, can only lead to disaster.

It is to Mr Mattis’s credit that the betrayal of Washington's Syrian allies in this case, and the global system of alliances in general, underpins his resignation.

Mr Trump keeps handing US enemies, including Russia, North Korea, China and now Iran, extraordinary and undeserved victories. It's no wonder his defence chief wants nothing more to do with it.

How much more the country as a whole can take is becoming an increasingly open question too.

With reckless incompetence at this scale combining with mounting scandals and criminal investigations, the answer may not necessarily be two more years.

Hussein Ibish is a senior resident scholar at the Arab Gulf States ­Institute in Washington