“It combines the cack-handedness of Bay of Pigs, the drama of Saigon in 1975 and shades of the Iran hostage situation in 1980.”  There you have Andrew Neil’s verdict on the humiliation of America in Afghanistan.

Some of those who aren’t following events there, and are too young to remember the Vietnam War, will nonetheless have picked up the comparison.  (“There’s going to be no circumstance where you see people being lifted off the roof of a embassy in the—of the United States from Afghanistan,” said Joe Biden, just over a month before exactly that took place.)

They will have seen montages of the two evacuations side by side.  If they are too young to recall Vietnam, they will also be too young to think back to the Bay of Pigs – and, perhaps, the United States’ hostage agony in Iran.  Of all three, that last is perhaps the one worth dwelling on most, and it was acute of Neil to make the comparison.

But this time round, it applies potentially to citizens of other countries – including Britain.  The Government is committed not only to bringing our nationals home, but also to implementing its Afghan Relocations and Assistance Policy (ARAP), which “is for Afghans who have supported British efforts in Afghanistan, for example interpreters and other personnel”.

It has four categories: high risk/imminent threat (urgent relocation), eligible for relocation by default (routine relocation), not eligible for relocation (other support offered) and special cases (case by case basis).  The scheme is being “delivered by the MOD in the new Afghan Threat and Risk Evaluation Unit (ATREU) in the British Embassy Kabul”.

That’s only the start. The Government is also prioritising the following groups: current or former Chevening Scholars, people with existing leave or an open application for student, work, and family visas, journalists and those who worked with British news agencies, Afghan government officials, officials working in counter-terrorism and counter-narcotics.

Plus, more broadly, members of civil society groups for womens’ rights, and employees of charities, humanitarian organisations and NGOs.  Ponder the pressure on our armed forces and civil servants – and the obstacles, first, to finding the people we want to assist and, second, to not assisting people we don’t want to assist.

The Government is playing it cool on claims that a person from Afghanistan on the UK’s no-fly list has been flown into Britain as part of the evacuation operation.  We don’t know the facts and so can’t reach a conclusion.  But it is all too easy to understand the potential for fraud and error.

“British officials at the airport in Kabul are warning of a spike in impersonations, forged documents and forged passports,” Sky News reports.  Add staff exhaustion to the mix, and there are formidable problems.  The logistical difficulties of carrying out the bigger resettlement scheme, for up to 20,000 Afghan refugees, will become clear in due course.

Many of those Britain is committed to helping will be a long way from Kabul.  Special forces are clearly active in the capital and elsewhere – the SAS has rescued some of its colleagues near Kandahar – but the details of their work are necessarily obscure.  This brings us to the Morton’s Fork on which the Government is impaled.

On the one hand, it seems desirable for the final date of evacuation to be pushed back from August 31 into September.  This would give our military and civilian personnel more time in which both to assess those seeking help and to find those to whom we want to offer it.  But on the other – crushingly – we aren’t in control.

The long trail of negotiations between America and the Taliban that Barack Obama tried to open, which Donald Trump concluded with the Doha deal, and which Biden has decided to implement has left the Taliban in control of “the facts on the ground”, or at least in Kabul.

A former Defence Minister yesterday reminded this site of an old military saying.  “The enemy gets a vote,” he said, “or in this case a veto”.  Here is the other prong of the fork.  For while a later departure date than August 31 would, in principle, allow us to evacuate more people it would also, in practice, endanger our troops, staff and others if the Taliban cut up rough.

Which they are threatening to do.  Remember, they have form: the Taliban offensive before which the Afghan Government collapsed was launched after Biden delayed the original departure date of May 1.  And what happens in the interim if ISIS or other suicide bombers strike at our Embassy?

One can see why Boris Johnson wanted to keep his options over before today’s virtual summit.  Another senior Tory yesterday floated sending a sizeable armed force into Afghanistan to further the evacuation.  But though this might be militarily feasible – Britain could doubtless assemble and deploy such a force, in time – it isn’t politically, unless we want to dive back into Afghanistan’s lobster pot.

Then there are the challenging logistics.  But the Prime Minister won’t be mulling such a plan today.  Instead, for all the fine words about “safeguarding the gains made in Afghanistan over the last 20 years – in particular on girls’ education and the rights of women and minorities”, he will concentrate on getting other countries to commit to providing aid and taking refugees.

Some say that, just as America bounced back after the Vietnam war and Iranian hostages, so it will again.  Maybe, maybe not.  Others add that all Biden is doing is closing a failed chapter in Afghanistan in order to concentrate American power elsewhere.  Again, maybe, maybe not – though if you were a country wavering between America and China, which would feel to you more like a rising force?

Whatever may happen in the medium term, the short-term is dominated by that deadline: August 31.  Perhaps it’s melodramatic to suggest a parallel between Britain’s presence in Afghanistan now and America’s hostages in Iran then.  But even if the evacuation goes as smoothly as possible, our exit can’t end well.

There was no social media in 1979, when those hostages were seized.  Prepare for a barrage of agonised tweets and posts from those campaigners for women’s rights, journalists, workers for NGOs, and just about anyone else terrified of the Taliban who has a social media account.  Ben Wallace is correct to prepare us for what’s coming: “not everyone will get out”.

If Britain ends its mission in Afghanistan on August 31 or before, events there will doubtless fade from the headlines afterwards. And there’s no evidence that the debacle is contributing to the Conservatives’ narrower though persistent poll lead.

But the powerlessness in Afghanistan thrust on us by Biden, and from which we can’t redeem ourselves as a lone actor, will continue to haunt the Government, frustrate MPs, and be felt more widely, as Taliban brutality is communicated to us in live time.  While our servicemen who fought there, present and former, ask the age-old question: what was it all for?