The bomb tore through an examination hall in Kabul on Friday, where students – mostly minority Hazara, mostly young women – were sitting a practice test in preparation for university. Thirty-five were killed, dozens more injured. An unspeakable human tragedy.

We don’t formally know who did it, but we can guess. Under the Taliban’s leadership, Afghanistan is a haven for terrorists. And the terrorists compete.

The Taliban is, in my judgement, indistinguishable from al-Qaeda. Its eyes are still firmly placed on international terrorism: a campaign of domestic terror within Afghanistan against ‘enemies within’ – be they former members of the internationally-recognised Afghan government, or religious minorities, or campaigners for liberty and women’s rights.

But the people who bombed this school are most likely Isis. The Islamic State was soundly defeated in Iraq, Syria and the Philippines, but it’s making a comeback in Afghanistan.

Once kept largely under the thumb of the United States Air Force, Isis’ resurgence began even before the Taliban took power: Joe Biden was operating on old information when he said that Isis was not a significant factor in Afghan affairs on 25 August last year. Less than 24 hours later, an Isis terrorist detonated a bomb outside Kabul airport, killing 13 American soldiers and at least 170 Afghans.

America was so impotent and its intelligence so bad by this stage that when it struck back at what it claimed was an Isis target on 29 August, it killed ten civilians instead – seven of them children.

The bombings have not stopped as America has retreated. Across Afghanistan, every week, Taliban soldiers are killed by Isis cells. Sometimes by gunfire, sometimes by bombing. More than monthly, mosques are targeted too.

Rahimullah Haqqani, a cleric close to the Taliban, was killed in a bombing on 11 August this year. He had been threatened and almost killed by Isis on numerous occasions. But now the Taliban is in power, they cannot protect him and everyone – all across the country – at once.

A week later, a mosque bombing in Kabul killed more than 25 worshipers, and the Sufi Amir Mohammad Kabuli. At the beginning of September, Mujibur Rahman Ansari, a cleric close to the regime, and more than a dozen others were killed in a mosque bombing in Herat.

It must be made clear: this is a greater frequency of bombings largely unseen after the US-led intervention since 2001. Nothing in the long 21-year history of international occupation and reconstruction matches this pace of terrorism and the level of its destruction.

It is clear that the Taliban cannot contain or defeat Isis. It is too loosely organised, too prepared to inflict constant, savage violence. For all their demand for international recognition and their pretence of legitimacy, the Taliban is still unprepared and unwilling to govern Afghanistan – it cannot do so even as well as the people it has overthrown. Its fighters are not policemen; they cannot keep order or administer local affairs. Its leaders cannot transmute their own fanaticism into a social contract.

The Taliban may realise that running a two-decade long terrorist insurgency may teach the members of an organisation about how to commit acts of terror, but not necessarily how to prevent others from perpetrating them on their watch.

They could not govern Afghanistan in good times. They know no economics. They are not legislators. But to believe they could do so at a time of near-starvation, with a vicious insurgency such that we see now ongoing, is impossible.

At the risk of irritating readers, let me say that this is mostly the fault of the world’s democracies, and of America in particular.

Not only did the United States hand Afghanistan over to the Taliban when there was no need to do so last year – by sabotaging the Afghan government’s capacity for self-defence – Joe Biden also made the Taliban America’s partners against Isis. He sought Taliban permission to evacuate Americans, and accepted their refusals and humiliating conditions as the price of doing business.

Britain, Canada and Germany are not innocent. Each left people unwilling to be abandoned behind, in the Taliban’s care. All of this was a devastating mistake. Allowing the Taliban and ourselves to believe that they could police the country, when their entire modus operandi was itself chaotic and insurgent, was a profound delusion. To assume that they could be partners, absurd.

As the bombs of Isis continue to go off across Afghanistan, we grow closer to knowing the cost of the West’s withdrawal. This is the price paid by the people of Afghanistan.

It’s something Afghans – and the westerners who will, in the future, be targets of terrorism launched from Afghanistan – will live to regret.