Leading article: Troops out. Unless...

It is no panicky reaction to sad news from the battlefield to say that the coalition's mission in Afghanistan is in deep trouble. It has no clear end point towards which to work – certainly none that is realistically achievable – and has been allowed to drift since the Taliban regime was deposed eight years ago.

That original intervention was morally and legally justified. The United States was entitled to demand that al-Qa'ida be brought to justice, and was entitled to the support of the United Nations when the Taliban refused to surrender Osama bin Laden. After the fall of Kabul, the war aims became more ambitious – for laudable reasons, which this newspaper supported at the time. The US-led coalition hoped to rebuild the civic and physical infrastructure of the country, with a military presence to maintain order, handing over in the medium term (that is, about now) to Afghan forces under the control of a democratic government.

Unfortunately, at the critical time when our forces could have created space for development, the coalition was distracted by the disastrous adventure in Iraq. And, because British casualties were amazingly low – only eight of our soldiers were killed before 2006 – Afghanistan slipped right down the list of concerns. It was not until British forces stepped up to the responsibilities shunned by other Nato partners and took over from the US in Helmand province three years ago that hard questions began to be asked.

For a while, it was possible to suppress the doubts. Afghanistan is a poor, tribal society from which foreigners have always retreated hurt. It was possible to suppose that history might be different this time. The US and UK are generally regarded favourably by Afghans. An opinion poll carried out in May found that 84 per cent thought "foreign assistance" was important in providing education, healthcare, roads, sewers and water supplies. But foreign forces are bound to become unpopular over time, and it should be evident by now that the Taliban cannot be militarily defeated – and certainly not by foreign forces. The Taliban may be disliked but they can draw on nationalist sentiment, which can only increase.

That same poll also found that 68 per cent wanted their government to "hold talks and reconcile with the Taliban".

It was possible, too, to think that something might be done about poppy farming. British officials spoke brightly of "diversification" into other crops, and Tony Blair briefly considered the more radical option, which this newspaper supported, of legalising opium production to produce medical opiates. But, as Patrick Mercer, the Conservative MP who briefly advised Gordon Brown, writes today, the poppy harvest in Helmand has almost doubled on our watch.

Mr Mercer proposes, however, one of the weaker arguments for the continuing coalition presence, that "too many lives have now been lost ... for this campaign to be abandoned". The Prime Minister made a slightly better attempt in his letter to select committee chairmen yesterday, but Raymond Whitaker's analysis exposes the flaws in the Government's case. At the level of the squaddie, the justification now falls back mainly on "helping the Afghan people" – and British soldiers are doing good work. But eight years on, they are still doing it.

The British people and their armed forces deserve better than this. Military action abroad cannot be sustained without solid support at home. We have severe doubts about whether our goals can ever be clear and whether they can be achieved by conventional warfare.

They certainly cannot be achieved if our troops fight with both hands tied behind their backs. If our troops are to stay in Afghanistan, they must have the equipment they need. But even if they had enough heavy-armoured vehicles and helicopters, the Taliban would find ways to attack them.

And commanders should have the numbers they need – as we report today, retired generals are aghast at plans to pull out 1,500 soldiers. But even if they had the forces they want, what they need above all is a believable cause for which to fight. The threat from al-Qa'ida is too tenuous to continue to justify this operation. It may be that the terrorists would be better fought by intelligence and special forces than by trying to turn Afghanistan into a stable democracy.

The time has come for Mr Brown to persuade us of the case all over again. If he cannot do so with far greater clarity and conviction, he must bring our troops home.