In recent days, and unsurprisingly, it has become common to hear the mournless rites being read for liberal interventionism. If anyone has opined publicly about Afghanistan in the last week - and plenty did - it was to regret our presence there and to wish us away. If ever an argument was being won by default this was it, especially since those making the case for quitting were far too exuberant to want to slow up and allow for the possible objections to their reasoning.

It was Condoleezza Rice, agitating for more Nato troops to be deployed in Afghanistan, who precipitated the current poison-ivy rash of isolationist critiques. This week in Lithuania Nato defence ministers are meeting to discuss finding 7,500 more troops to reinforce the existing 42,000, and last week there was a run-in between the Americans and the Germans over whether Bundeswehr resources could be sent to the dangerous south - a spat that the Bundesmedia seemed to enjoy a bit too much.

To which many resonant voices here were raised to make this point: we don't have the men, and even if we did we shouldn't send them; in fact we should start talking about withdrawing the ones we've got because the whole thing is broken and cannot be mended. “We British,” wrote Matthew Parris on Saturday on these pages, “are at our limit and losing confidence in our usefulness.” Independent reports speak of a danger of failure and a “weakening international resolve”, and the few gains of our continued presence - “a few new schools and roads in the north”, according to Simon Jenkins in The Sunday Times - are insufficient to stop the country fragmenting.

And it is worse than that, they imply, because most of the problems that exist we have ourselves provoked and indeed spread to neighbouring Pakistan. “To have set one of the world's most ancient and ferocious people [the Pashtuns] on the warpath against both Kabul and Islamabad takes some doing. But Western diplomacy has done it,” says Jenkins; though why the Pashtuns are any more ancient than the rest of us, and why it should be so surprising that “one of the world's most ferocious peoples” might be relatively easily provoked, he doesn't explain. The tribal areas of Pakistan and Afghanistan, he argues, should have been left alone.

There has, of course, to be another logical step taken here, and this is it: for what cause have these bloody errors been committed? The cause of combating terror. But terror is an overblown threat, they say, exaggerated by men like Bush and Musharraf: terror kills few in the West and is generally contained by good policing. Our troops are making things worse. Rather than a War on Terror, we might do better to talk of a musing on terror, or - at worst - the tiff with terror.

In the current circumstances of the failure of the opium strategy, the bloody fighting in Helmand, the row inside Nato and the argument about Paddy Ashdown's unacceptability to Hamid Karzai, the Afghan President, much of this pessimism seems appropriate. But if we are to follow its dictates, its proponents should do a better job of spelling out what it means. Anyone who still favours a military presence is easily decorated with the order of the armchair commentator, but let us see what other commentators are prepared to sit through.

Canada has already threatened to pull out its troops from Kandahar province in a year's time if other Nato countries don't contribute more. We must assume that if Britain were to begin to talk about a draw-down, then Canada would carry out this threat. British forces would then be exposed in Helmand and, presumably, would also withdraw. Let us suppose that an angry and abandoned US follows the “lead” offered by its allies, and itself pulls out, leaving itself only an air-to-ground interdiction capability.

Here are the likely consequences of such a pattern. The Afghan Government would collapse, to be replaced by an overt civil war fought between the Taleban and local governors in the various provinces. A million or more Afghan refugees would again flee their country, many of them ending up in the West. Deprived of support from the US, as recommended by our commentators, President Musharraf or a successor would effectively withdraw from the border regions, leaving a vast lawless area from central Afghanistan to north central Pakistan. Al-Qaeda and other jihadists would operate from these areas as they did before 9/11. This time these forces - already capable of assassinating a popular democratic politician - would seriously impact upon the stability of Pakistan, which is a nuclear state.

Jihadists everywhere, from Indonesia to Palestine, would see this as a huge victory, democrats and moderates as a catastrophic defeat. There would hardly be a country, from Morocco to Malaysia, that wouldn't feel the impact of the reverse. That's before we calculate the cost to women and girls of no longer being educated or allowed medical treatment. And would there be less terror as a result?

We have been here before. After the Afghans managed to defeat the Russians, the Yanks - and everyone else - left Afghanistan alone, to be swallowed up by the Taleban. Who then let Osama bin Laden in. It wasn't us who provoked the ferocious Pashtun in 2001, it was their Mullah Omar who gave sanctuary to the topplers of the twin towers. Many of bin Laden's people had themselves been radicalised by the failure of the West - in another non-intervention - to prevent Serb atrocities against Bosnian Muslims.

Whatever the failures of Western policy - which have usually been about doing too little, not too much - they will not be dealt with by the creation of a new myth of non-interdependence. Just as the genocide in Darfur has refused to confine itself within the borders of the Sudan, but has now destabilised neighbouring Chad, so anything that happens in Pakistan or Afghanistan, whether we cause it or not, will come back to us in the shape of fleeing people, apocalyptic ideologues, weapons proliferation and the export of terror.

Fortunately, it isn't just David Miliband who recognises this. Today America may decide that the next presidential election will be between John McCain and Hillary Clinton or Barack Obama. All three recognise that America must continue to be the ideological and physical arsenal of democracy. Thank God.

Times Recommends

• Fundamentalist brew: faith and power
• Big guns wheeled out for the marginals
• Wanted: new merchants of light