Donald Trump’s rash and foolish decision to pull the remaining US ground troops out of northeast Syria is a shocking betrayal of the Kurdish forces that were instrumental in destroying the Islamic State “caliphate”. It opens the way for a vicious, protracted struggle between the Kurds and Turkey’s military, which is poised to cross the border. And that in turn presages more civilian suffering in a country that has seen far too much during the past eight years.

Trump’s impromptu order was taken against the advice of his generals and diplomats and without prior consultation with allies such as the UK that have forces in the field. It came following a telephone conversation with Recep Tayyip Erdoğan, Turkey’s president, on Sunday evening. Trump tried last year to withdraw US forces but was thwarted at the time. Now he has got his myopic, capricious way. Erdoğan has been pushing for months to create what he terms a “safe zone” on Syrian territory 20 miles deep by 300 miles long. For him, too, altruism is not a motive.

The US’s contemptible retreat and Turkey’s illegal land-grab represent the final collapse of western policy in Syria

Erdoğan has three aims, all problematic. One is to force the Kurdish-led Syrian Democratic Forces (SDF), which comprise about 60,000 fighters, away from Turkey’s southern border. Erdoğan vilifies the SDF as terrorists in cahoots with the PKK – the Turkey- and Iraq-based Kurdistan Workers’ party that Ankara has been fighting for decades. The terrorist tag is not remotely accurate. But demonising all Kurds as enemies of the state is a familiar tactic used by Erdoğan to bolster his divisive, dictatorial nationalist agenda.

Second, Erdoğan has plans to return, by force if necessary, many if not most of the 3-4 million Syrian refugees who have entered Turkey since 2011. His ruling AKP party and its ultra-nationalist allies have encouraged growing anti-migrant sentiment, effectively expelling these refugees from the larger cities. They’re happy that blame for Turkey’s faltering economy, high unemployment and social tensions can be directed at Syrians and other foreigners rather than at their own corrupt, repressive and incompetent management.

Erdoğan is also badly in need of a political and strategic success after a series of domestic reverses, including the AKP’s humiliating loss of Turkey’s two biggest cities, Istanbul and Ankara, in recent elections. Talk grows of an end to the Erdoğan era – something he cannot abide. Erdoğan also hopes to correct the mess he has previously made of his Syria policy. He initially courted the Damascus regime after 2011, then turned against it, then colluded with Russia and Iran – Bashar al-Assad’s main backers. That put him at odds, latterly, with Washington, Turkey’s key Nato ally.

In the gullible, geopolitically ignorant Trump, however, Erdoğan has found a friend and like-minded instinctive authoritarian. It’s plain Trump would rather give Erdoğan – a co-collaborator with his Moscow mate, Vladimir Putin – what he wants than keep faith with the Kurds. This unlovely, three-way gangster-partnership now presages a world of problems in Syria. One possible consequence is that the Kurds, their loyalty and sacrifices again repaid with betrayal, will cut a self-preservatory deal with Assad – or, alternatively, that their thwarted drive for an independent state will revive.

Dangerous, too, is the boost the US retreat potentially gives to Isis. The jihadists, down but not out despite Trump’s self-serving claims of victory, are already said to be regrouping in northern Iraq. Fears grow that detention and refugee camps in eastern Syria where tens of thousands of Isis militants, supporters and families, including about 2,000 foreign fighters, are held under Kurdish guard may be compromised – and could become recruiting centres for Isis redux. The White House says the Turks will take charge. Given their record of covert dealings with jihadists, that’s a big, reckless, overly optimistic gamble.

Renewed fighting in northeast Syria, potentially spreading westwards to areas such as Afrin – seized in an earlier Turkish incursion – and even to besieged, war-ravaged Idlib, threatens yet another humanitarian disaster. What Turkey now proposes, with Trump’s blessing, amounts to the forcible repatriation of hundreds of thousands of defenceless civilians into what may soon be, or already is, a war zone. Don’t be fooled by US-Turkish spin. It’s not safe. And it’s not right. This is a war crime in the making.

Latest betrayal of Kurds risks undermining defeat of Isis Read more

Viewed more broadly, the US’s contemptible retreat, and Turkey’s illegal land-grab, represent the final, miserable collapse of western policy in Syria. It marks the abandonment of any remaining pretence that the US and Europe have the will, the commitment and the humanity to rescue the Syrian people from a murderous regime, make good on the reform promises of the Arab spring, and create a viable path to democratic self-governance.

Even more so than Iraq after the 2003 invasion, Syria has become the epic failure of our age. Thanks to those geostrategic mobsters Trump and Erdoğan, with a big assist from smarter-by-far Putin, the country faces informal partition into highly conflicted, de facto Turkish, Iranian, Russian, Israeli and jihadist areas of influence and control; a repugnant regime in Damascus of mass murderers and assassins will escape justice; and the dream of an inclusive democracy is dashed.

For Syrians of all backgrounds and beliefs, a paradoxically permanent instability is the new normal. And, since you ask, is there any point demanding that the UK and Europe take a stand and, at last gasp, resist this foul denouement? Not really. It’s too late now. The Syrian failure is printed on all of our foreheads. It indelibly shames us all.

• Simon Tisdall is a foreign affairs commentator