The conflict engulfing north-east Syria is a wholly avoidable disaster. It was widely foreseen. It could, and should, have been prevented. Responsibility lies principally with Turkey’s bellicose president, Recep Tayyip Erdoğan. But many others share the blame, including a criminally incompetent Donald Trump, Islamic State jihadists, who previously destabilised the area, and the international community, which has failed, over the course of eight bloody years, to halt Syria’s civil war.

The terrifyingly indiscriminate Turkish artillery barrages and air strikes directed at towns and villages in Kurdish-held areas along the border shame those who ordered them. Erdoğan’s claim that his forces are only targeting terrorists is given the lie by the rising toll of civilian deaths and injuries. Aid agencies have evacuated. Hospitals have closed. The UN says about 100,000 people have fled so far. With Turkey rejecting calls to halt the offensive, it could all get much worse.

This is a calamity foretold. Turkey has longstanding, legitimate border security concerns. It believes the Kurdish militia that controls north-east Syria is in league with its old foe, the Turkey-based Kurdistan Workers’ party (PKK), which the US and the EU, like Ankara, regard as terrorists. Erdoğan had been threatening military action east of the Euphrates for months. Only the presence of US troops stopped him.

US officials say an agreement with the Turks was in place, providing for joint border security patrols. But this was not enough for Erdoğan. His impatience arose not from the immediacy of the terrorist threat, which he often exaggerates, but stemmed, at least in part, from his need for a political “win” after recent election setbacks and from rising rightwing nationalist pressure to repatriate Syrian refugees to a Turkish-controlled “safe zone”.

Who could blame Kurds, with their homes under attack, if they abandoned the detention camps to resist the invader?

It is at this point that Erdoğan’s agenda converged with Trump’s visceral aversion to “endless” foreign wars and the impeachment furore in Washington. When Erdoğan phoned last Sunday evening, demanding that the US lift its veto on intervention, Trump saw a chance to both bring the troops home and distract attention from his Ukraine shenanigans.

Official assertions that Trump did not give Erdoğan a green light are pure eyewash. The White House statement issued after the phone conversation makes clear this is exactly what happened. And yet, on one level, this outcome is unsurprising. Erdoğan and Trump are two of a kind: unscrupulous, instinctively authoritarian leaders ever ready to bend the truth. Neither can be trusted.

These two men have something else in common. They do not understand, nor do they sufficiently care about, the consequences of their actions. Trump seems to have been genuinely taken aback by the storm of criticism, including from Republicans, which greeted his decision to pull back US troops. He was rightly lambasted for betraying America’s Kurdish allies and helping Russia, Iran and Bashar al-Assad’s Syrian regime. Fears were raised that Isis jihadists held under Kurdish guard might escape.

The fate of detained Isis fighters, totalling about 10,000 across northern Syria and Iraq, is an issue to which Erdoğan, too, has plainly not given enough thought. He says Turkey will ensure they do not abscond. But his unimpressive ground forces, still thrashing about on the border, cannot be counted on to fulfil such pledges. Already there are reports of an Isis prison break in a town under Turkish bombardment and two Isis suicide bombings.

Trump repeatedly, untruthfully, boasts that Isis was defeated on his watch. The “caliphate” is destroyed, but the jihadists still pose a threat, as a new International Crisis Group study shows. There are persistent reports that the organisation is regrouping. Trump and Erdoğan have potentially assisted this process. Who could blame Kurdish fighters, with their homes under attack, if they abandoned the detention camps and went to resist the invader?

The international community is at fault, too, for failing to establish a process for bringing Isis terrorists to justice. Leaving them, their families and supporters stuck indefinitely in desert camps was never going to work. Western countries, including Britain, have mostly dodged their responsibilities in this regard, concerned that jihadists who hold British or European citizenship could be freed by domestic courts for lack of admissible evidence. To address this problem, they should consider the creation, under UN auspices, of an international criminal tribunal for counter-terrorism.

Sadly, as the entire history of the Syrian war suggests, the chances of such international collaboration actually happening are all but non-existent. The UN security council, debating Turkey’s action, could not even agree a joint statement, due in part to the usual Russian obstructionism. The EU will discuss it at this week’s summit. Expect little more than stern words. Nato is just looking on. Meanwhile, Trump blusters about sanctions, as if it all had nothing to do with him.

Pity the people of northern Syria, bombed and blasted from their homes by a ruthless autocrat who should, if there were any justice, face a war crimes tribunal. It seems there is no helping them. What an outrage. No wonder the world is in such a mess.