Gordon Brown said yesterday that Britain’s military commitment to Afghanistan was undiminished. But an unpopular Government is in a weak position to expound a coherent purpose to the campaign. Those, including The Times, who have supported the intervention since the outset must respond to a question that has become increasingly urgent. Were we wrong?

Fifteen British soldiers have been killed in Afghanistan in the past ten days. In all, 184 British soldiers have died since the US-led invasion in 2001. For the public, never mind grieving family and friends, these deaths are especially disheartening when there is no indication of when Britain’s military commitment will end.

Nick Clegg, the Liberal Democrat leader, spoke this week of the danger that the mission in Afghanistan would become a quagmire if the Government did not provide proper resources and a political strategy. In US debate, “quagmire” is forever associated with the disastrous military campaign in Vietnam 40 years ago. Its principal architect, Robert McNamara, died this week having long regretted his decisions.

Mr Clegg has done a public service in forcing the defenders of the Afghan intervention to consider whether the means are sufficient to the ends, and what those ends realistically are. So let it be stated plainly: the cause in Afghanistan is just, the fight is necessary, and a withdrawal would damage British security as well as betray a country to which we have an obligation.

The cause is just because of the nature of the adversary: thugs and fanatics who repressed Afghanistan and allowed a transplanted Islamist group to plot the attacks of 9/11. Whereas the US in Vietnam contended with a deep and enduring campaign for national independence, the Taleban are a movement to crush a nation. When they controlled the country, they slaughtered thousands of Shia Hazaras on the ground that they were a supposedly heretical branch of Muslims. It has been well said that the campaign to oust them is the first instance in recorded history of bombing a society out of the stone age.

The cause is necessary because the US and its allies are acting against those directly responsible for the murder of thousands of civilians of many nationalities on the American mainland. The notion that when al-Qaeda was based in Afghanistan it formed a separate entity from the Taleban is absurd. They have a symbiotic relationship grounded in a common malevolent ideology. Afghanistan is no longer sanctuary for al-Qaeda but it will revert to that role if the US-led coalition fails or abandons the task.

If Afghanistan were abandoned, the threat to Western security would be intensified by the effect on Pakistan. A weak Pakistani Government allowed the Taleban in effect to use the Swat Valley as a means of destabilising Afghanistan. If constitutional government — however imperfect — in Afghanistan falls, then the country will immediately become a base for the equivalent movement in Pakistan. The notion that an Islamist takeover of a nuclear-armed state would be of no security interest to Britain is incredible.

The campaign in Afghanistan is crucial. It has been advanced by British servicemen of extraordinary courage and real heroism. The failures in the campaign have been on the home front: the British Government has been dilatory and uncertain in making the case for the war. Nor has it provided sufficient armoured vehicles for the troops already there or adequate and consistent numbers of troops on the ground to establish the security that is the foundation for building a new nation in Afghanistan. The campaign for Afghanistan must be won. And it is time the Government started doing a better job of fighting for it at home.