ONLY last week the White House reckoned it could be months before the Taliban fought their way across Afghanistan and reached the city limits of Kabul. In the end it took mere days and hours to make the unthinkable real.

Twenty years after the US, with the help of the UK, invaded Afghanistan in response to the attacks of 9/11, the clock has been turned back and the roles of victor and vanquished reversed.

All those lives lost among civilians, British soldiers, and American troops, and for what? The promises set down, the expectations raised, the billions squandered. All to dust.

As MPs return to Westminster on Wednesday to discuss what one of their number, Tory MP Tom Tugendhat, has called the single biggest foreign policy disaster since Suez, there will be many questions demanding answers. Chief among them should be whether the speed and manner of the American and UK withdrawal hastened the Taliban takeover and made an already desperate situation worse.

There has been an obvious scramble for cover in London and Washington since President Biden led the way with his initial announcement of US withdrawal in April. Boris Johnson fell in soon after, regretting that there was no alternative but to follow the Americans out the door.

Both President and Prime Minister might have thought the way ahead, though fraught with risk, was clear. The plan was to push back against any criticism that withdrawal was happening with the job half done, get the soldiers, diplomats and others out in an orderly fashion, and, finally, move on. Reality has proven to be more difficult. But how much blame and responsibility should the current US and British administrations shoulder for this?

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In easier for Mr Johnson to distance himself from blame than it is for some others. The bombing of the country, followed by the invasion of Iraq, was a Bush-Blair production after all. When 9/11 happened Mr Johnson was a lowly backbench Opposition MP. Nothing to do with him, guv. That does not absolve him from acting now, however.

What of Mr Biden? In theory he can also argue that the original intervention happened on someone else’s watch, and he is acting decisively to put an end to it. With a recent poll showing 73% of Americans supporting the decision to withdraw, the will of the American people was clear.

Yet as the situation in Afghanistan became more chaotic, criticism at home mounted. The media began raising the ghosts of Saigon. Those helicopters, that panic. By Saturday, President Biden felt it necessary to clarify his position – never a sign that all is well.

Saying he had “inherited a deal cut by my predecessor” (his contempt for the man barring him from using Donald Trump’s name), Mr Biden insisted he had been left with two options. He could follow through on Mr Trump’s deal to withdraw US forces, or he could double-down and “send more American troops to fight once again in another country’s civil conflict”.

But one year or five years, it would make no difference if the Afghan military “cannot or will not hold its own country”.

His conclusion was brutally frank. “I was the fourth President to preside over an American troop presence in Afghanistan – two Republicans, two Democrats. I would not, and will not, pass this war onto a fifth.”

There is more at stake here for the President, and to a lesser extent for the Prime Minister, than any initial loss of face. First, Mr Biden promised America that under his presidency politics would become boring again. The crisis in Afghanistan is anything but that.

Secondly, this was the President who promised that America was back as a global leader, ready to show the way on everything from tackling climate change to standing up to China and Russia. If America is seen to give up on Afghanistan, albeit after two decades of trying to build a stable democracy there, what value its words and guarantees on other matters?

Thirdly, there is the question of what Mr Biden was trying to do in acting so swiftly on Afghanistan. Was it an attempt to head off an attack from Donald Trump and court the Republican right? If so it was misjudged. In trying to please the right he is storing up further trouble on the left of his own party.

For both President and Prime Minister, much depends on what happens in the coming hours and days in Afghanistan. While initial reports suggest the takeover has been peaceful, there is no guarantee the situation will remain that way. Given the track record of the Taliban and their murderous progress so far, Afghan citizens, and women in particular, have every right to be terrified about what comes next.

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Nor should it be forgotten that the main reason for ousting the Taliban in the first place was that they had made the country a haven for Islamist terrorists. What is to stop them setting up camp again if they have not already done so?

Many a best laid plan by foreign invaders lies buried in Afghanistan. The Blair-Bush folly is another to add to a sorry list. In this case, however, the consequences of failure are unfolding before us here and now. The west can walk away from Afghanistan, but it cannot in all conscience look away.