Leading article: Why we must leave Afghanistan

One by one over the past eight years, the arguments for the continued presence of Nato troops in Afghanistan have fallen away. The last one, which held us back until now from calling for withdrawal, was the need to police the Afghan election in August. That election process is now over: last week the president's main opponent pulled out, and Hamid Karzai was formally re-elected. That is not a happy outcome. For British soldiers to be deployed in support of a president whose position is bolstered by ballot-rigging tips the balance of our view from reluctant backing for the mission in Afghanistan to regretful opposition.

The Independent on Sunday is proud of Britain's armed forces, and has led the way in demanding that the Government honour – on behalf of the British people – its side of the military covenant, to provide troops with the equipment that they need and the support that they and their families deserve. On this Remembrance Sunday, we reassert our belief that our forces are entitled, above all, to clear and believable war aims.

This newspaper was never keen on the Afghan intervention, although we did distinguish it sharply from the Iraq war, which we opposed strongly throughout. When Tony Blair said, before the bombs began to fall, "We must bring Bin Laden and other al-Qa'ida leaders to justice" and "ensure that Afghanistan ceases to harbour and sustain international terrorism", we pointed out that the terrorist threat to Britain from al-Qa'ida training camps was tenuous.

Once the conflict began, we asked: "What is this war for?" Although we never received a satisfactory answer, we welcomed the fall of the Taliban and reluctantly accepted Mr Blair's argument, made with his trademark persuasiveness, that the best protection against their return was to help rebuild the country. Thus the mission crept from bringing mass murderers to book to fostering democracy, female emancipation and winning the battle against drugs. Those are worthy aims, but eight years on we have made limited progress. In the meantime, British forces, which had borne few casualties until then, were deployed in 2006 to Helmand province.

It is not so much the casualty rate, however, but the lack of progress that should demand a re-examination of our policy. Gordon Brown's speech last week did not deliver the review needed. It contained the glaring contradiction between the claim that our troops are needed in Afghanistan "to keep the British people safe" and the warning that, if Mr Karzai's government fails to clean up its act, it will have "forfeited its right to international support". If you believe that our mission in Afghanistan makes British streets safer, then its continuation should not depend on Mr Karzai. If, on the other hand, you believe, as Kim Howells, the former Foreign Office minister, said last week, that any remote or long-term effect on British streets is outweighed by the propaganda gain to jihadist ideology of our "occupying" a Muslim country, then Mr Karzai's shortcomings give us another reason to get out.

There are, of course, still good reasons to stay, although they are secondary. The Afghan people do not want foreign troops to leave until security is better. But the longer we are there the more our forces provide target practice for jihadists and grievances for nationalists to turn to jihadism, as Patrick Cockburn argues so forcefully today.

A second reason for staying is that our withdrawal could undermine Barack Obama, whose leadership is needed in the world. But we have left Iraq while the US stayed. In any case, as we report today, the US is keen to move British forces away from being a political target.

Ultimately, we should make a British decision in the British interest. And that decision should be to wind down combat operations over a period – say, by Remembrance Sunday next year – and to restrict the mission to training the Afghan army and police force. Special forces operations should continue, especially on the Pakistan border, to disrupt any attempt by al-Qa'ida to return. But beyond that it is time to act on the observation of David Miliband, the Foreign Secretary, that there can be no military solution in Afghanistan.

It is time, on this solemn day on which we remember the sacrifice of those who gave their lives for our freedom and security, for a change in policy. It is time to say that this war is ill conceived, unwinnable and counterproductive. It is time to start planning a phased withdrawal of British troops.