Kabul has fallen. What can I say that doesn’t sound trite. Listen to the ex-solder Tom Tugendhat, who served in Afghanistan, speaking in the House of Commons, and I’m minded to hold back.

But there is one question I keep on asking myself, the big question, the ‘why’ question. Why did Joe Biden allow it to happen?

I thought I’d check what Barack Obama has to say (in ‘A Promised Land’) about the discussions they had within his newly-elected administration in 2009. The USA, he writes, already had 30,000 troops there, plus 10,000 troops from other countries, and the military were pressing for the deployment of another 30,000. Secretary of Defence Robert Gates warned of unintended consequences that could follow from rash actions.

In Obama’s words: ‘Unless we established limited and realistic objectives, he [Gates] told me, “we set ourselves up for failure”.’

Obama continues, ‘Among the principals only Joe Biden expressed his misgivings. He had travelled to Afghanistan on my behalf during the transition and what he saw and heard on the trip – particularly a contentious meeting with Karzai – had convinced him that we need to rethink our whole approach to Afghanistan…he saw Afghanistan as a dangerous quagmire and urged me to delay deployment, suggesting it would be easier to put troops in once we had a clear strategy as opposed to pulling troops out after we’d made a mess with a bad one.’

Biden has been by his lights entirely consistent. American strategy over the twelve years since he became Obama’s vice-president has been focused on nation-building, and putting that new nation on a secure basis, and guaranteeing that security. That simply hasn’t happened. The Taliban never went away. Warlords guarded their patches. Bribery remained endemic. Kabul, with many of the trappings of a big Western city, was the exception. And why should we have ever supposed that an Afghan government army drawn largely (I’m assuming) from outside Kabul would ever stand and fight against a single-minded and often brutal Taliban insurgency?

China and the ex-Soviet republics lie to the north. Taliban-sympathising Pakistan to the east, Iran to the west. What chance was there of guaranteeing Kabul, let alone Afghanistan, a Western-style democratic future?

Only if the US retained a big military presence – and the American public, 70% in one poll I’ve seen, wanted out.

Jon Sopel on the BBC website suggests the Americans could have delayed their withdrawal to the winter season, when no-one fights in Afghanistan. But that wouldn’t have made much difference in the end. Kabul would have fallen.

But not in such a catastrophic fashion.

Did Biden really believe the Afghan army would put up strong resistance? Was he just badly advised? Is this just another, and terrible, example, of that American insensitivity, that lack of awareness, which has so bedevilled its foreign policy ventures since World War Two?

This one matters. Women in Afghanistan face an uncertain future. It may be a terrible future. Kabul is far more than another political catastrophe. It could be a humanitarian catastrophe of an extreme kind.

Big decisions need fallback positions. Halting the withdrawal, or maintaining full air cover for the Afghan army, at the very least. All or nothing is no strategy.