In the week since Donald Trump’s fateful phone conversation with Recep Tayyip Erdoğan, the US has entirely abandoned the Kurds, its most effective allies in the Middle East, and with them a Syria strategy that was five years in the making.

The Islamic State flag has been raised once more and the last vestige of US credibility as a reliable partner lies crushed under Turkish tank tracks. It has arguably been the worst seven days for US foreign policy since the invasion of Iraq.

Administration officials have been under orders to deny that Trump gave Erdoğan a green light to invade north-eastern Syria, despite all the indications to the contrary. After the Turkish leader announced his intention to invade, Trump invited him to the White House, one of the most coveted rewards a US president can bestow. And even as his aides are instructed to lie on his behalf, Trump continues to flash a green light on Twitter, and not just to the Turks.

While echoing Turkey’s view of the Kurds as terrorists, Trump declared: “Others may want to come in and fight for one side or the other. Let them!”

Facebook Twitter Pinterest Mourners attend a funeral for the Kurdish political leader Hevrin Khalaf and others, including civilians and Kurdish fighters, in Derik, Syria. Photograph: Delil Souleiman/AFP via Getty Images

Those “others” include the Assad regime, and its Iranian and Russian backers, to whom the Kurds have appealed for their own salvation, as their US partners pack their bags and leave. A close, very personal and highly effective partnership between US (as well as British and French) advisers and Kurdish fighters in the Syrian Democratic Forces (SDF), which took nearly five years to forge, has been totally undone in a few days.

Since the US started dropping supplies to besieged Kurdish fighters in Kobane in late 2014, that partnership succeeded in rolling back Isis, rooting it out from one stronghold after another. At the same time, it held the regime at bay across a significant swath of Syria.

These were Washington’s limited objectives in Syria, inherited from the Obama administration. And they were achieved at very little cost to the US. The price was paid almost solely by the SDF, which lost 11,000 fighters.

Regime forces are moving northwards into areas that were in the US sphere of influence just a few days ago, at the invitation of the SDF, desperate for assistance in the face of Turkey’s murderous Syrian proxy militias. At the same time, Isis detainees are escaping from SDF detention facilities and the movement is reconstituting. The speed of the unravelling has been breathtaking.

Trump has played down the Kurdish relationship as purely transactional. They had been given a lot of money and equipment, he pointed out. But it was not transactional for the soldiers who fought at each others’s shoulders, and the civilians killed as a result of the Trump-Erdoğan understanding. The US has let the Kurds down before, but Trump’s sheer callousness has made it hard to imagine that this betrayal will be forgiven in the foreseeable future.

Facebook Twitter Pinterest Donald Trump. Photograph: UPI/Barcroft Media

The US military is also unlikely to forget being forced to cut and run, and watch a respected ally being crushed. There is considerable anger being reported from junior officers in the field and the generals back in the Pentagon. The next time US soldiers go looking for local partners to act as ground forces, it will be much harder. Who would fight with America now?

After three years of Trump, that is an increasingly difficult question to answer. The president has lavished praise on adversaries such as Vladimir Putin and Kim Jong-un, while insulting allies in Europe and Asia who he has convinced himself are ripping off the American taxpayer.

The flip side of the impeachment scandal is that while the US president was seeking to force his Ukrainian counterpart, Volodymyr Zelenskiy, to dig up dirt on Trump’s political rival, he was paying no heed to the supposed priority of his own administration, to bolster Ukraine in the face of the Russian threat. That does not seem to have been a factor for him.

There are even questions about Nato’s survival if Trump wins re-election next year. Erdoğan’s actions have raised doubts about Turkey’s continued membership, while Trump sees the alliance as a club of freeloaders exploiting the US. Until recently he was boxed in by a phalanx of his own officials with traditional Republican pro-Nato views. But the so-called “adults in the room” have dispersed one by one. Trump, increasingly unmoored, convinced of his own “great and unmatched wisdom”, is just improvising, calling foreign leaders and making decisions affecting millions of people.

What guides those decisions is obscure. The sprawling, ramshackle Trump business empire means he is rife with conflicts of interests every time he talks to Erdoğan or Putin or Mohammed bin Salman. Many of those conversations happen with few if any witnesses and any permanent record, as we have discovered in the course of the Ukraine saga, is locked up in a top secret system.

Facebook Twitter Pinterest Turkish soldiers drive American-made M60 tanks in the town of Tukhar, north of Syria’s northern city of Manbij. Photograph: Aref Tammawi/AFP via Getty Images

The descent of US foreign policy into chaos is a reflection of a broader failure of its political system. The constitution is supposed to give Congress a decisive say in treaties and the declaration of war, just as it is supposed to stop the US president from receiving emoluments from foreign powers.

None of that is functioning, mostly because Trump has a demagogic hold on the Republican party, whose members fear political obliteration if they dissent. And the president is gambling that the courts have been sufficiently packed with Republican loyalists over the decades that he feels he can afford to flout the law.

At each new demonstration of the president’s venality and volatility, there are predictions that the tide is about to turn, even Republicans have had enough and the republic is about to reassert itself. Such predictions may eventually come true, but it may well be too late for US credibility as an ally. It is already too late for the Kurds.