Announcing the sacking of General Stanley McChrystal, the top US commander in Afghan­istan, and his replacement by General David Petraeus, President Barack Obama said: "This is a change in personnel, not a change in policy." But the president is wrong. There needs to be a change in policy before it is too late and while the coalition forces are still able to negotiate from a position of relative strength and influence.

The Americans and the British cannot win their war in Afghanistan; the Taliban cannot be defeated militarily. The Afghan people have demonstrated throughout their long history that they will resist all attempts to be ruled from abroad or to have western-style puppet governments imposed upon them. Occupying forces from the British in the 19th century to the Soviet Union in the 20th century have demonstrated similarly that they do not have the patience or stamina to win a war and then secure a lasting peace in that blighted country.

There is widespread recognition among the lesser coalition partners that this latest Afghan war cannot be won on the battlefields. One by one, our allies are withdrawing or announcing their intention to withdraw. The Dutch will leave by the end of the year; the Canadians and Danish will follow in 2011. President Obama himself has signalled his intention to begin drawing down troops from as early as July 2011, and David Cameron has said that he would like Britain to have left altogether by 2015.

On 29 June, the bodies of the latest British soldiers to have died in the conflict were brought home. Once more, the crowds were out on the streets of Wootton Bassett to honour the fallen. More than 300 British troops have died in Afghanistan since the war began in the aftermath of the al-Qaeda attacks of 11 September 2001. Over 100 have died in the past year alone - an intensification of losses that bespeaks the failure of the policy of counter-insurgency.

Britain has 10,000 troops in Afghanistan; the Americans have roughly 100,000. The US has lost 1,000 troops; thus, the British losses, proportionally, are greater. Yet there is little recognition in American society of these British sacrifices or of the role that our troops are playing. In Michael Hastings's controversial profile of General McChrystal in Rolling Stone magazine, revelations from which led to the general's sacking, there was no mention of the British presence. The implication was that the Americans are in this on their own.

That may be the perception in the US, but the reality for the British troops on the ground is starkly different. The question that is being asked - even by those who once supported the occupation and its aims - is why our soldiers are continuing to die in a war that cannot be won. There was much bluster from Gordon Brown and others, to the effect that we were fighting in Afghanistan so that our people could walk safely on the streets of Britain, free from the threat of Islamist terrorists trained there.

This is nonsense. Before the invasion of Afghanistan and the subsequent illegal Iraq war, Britain was not under threat from al-Qaeda and even less so from indigenous terrorists. Rather, Tony Blair's wars radicalised young British Muslims, just as the educated, middle-class American citizen Faisal Shahzad, who failed in his attempt to launch a car-bomb attack on Times Square, New York, in May, was radicalised by deaths of civilians in US Predator-drone attacks on north-western Pakistan.

Our political leaders remain in denial. Listening to Harriet Harman, the hawkish acting leader of the Labour Party, discuss Afghanistan is to be reminded of the calamity that befell Labour during the years of Mr Blair's foreign policy misadventures. At least Ms Harman has not displayed the spectacular ignorance of the new Defence Secretary, Liam Fox, who said recently: "[We are] not in Afghanistan for the sake of the education policy in a broken, 13th-century country." In fact, what is required of the coalition in Afghanistan is precisely more investment in development projects, in education, road construction, irrigation, energy and health.

Afghanistan has been ravaged by war for more than three decades. The country has been under occupation of US-led Nato forces for the past nine years. The western-installed government, led by Hamid Karzai, is corrupt. The Taliban are barbaric and reactionary. Yet the Americans and British must not cut and run in despair. They must seek a negotiated political settlement immediately - and that means talking to and negotiating with the Taliban. The alternative is defeat for the allies and perpetual war for Afghans.