Leading article: The public mood is shifting, but the mission must push on

The Afghan surge has resulted in a harrowing increase in British military casualties. Fifteen of our troops have been killed in the past 11 days. Britain has now lost more soldiers in the country than in Iraq. And, perhaps unsurprisingly, the public mood at home has shifted suddenly over this war. The families of some service personnel are beginning to question the purpose of this mission. And the voices of those demanding withdrawal are growing louder.

It is healthy that Britain's military strategy in Afghanistan is the subject of vigorous political and public debate. There was too little scrutiny of the unfocused British deployment in Helmand in 2006. It was never clear whether the objective three years ago was to kill the Taliban, eradicate the opium crop or boost reconstruction. That haziness proved a disaster. In the end, none of these goals were accomplished.

It is also right that a forensic light is being shone on whether our troops have sufficient numbers and the right equipment to do the job being asked of them. The call of the former defence secretary, John Hutton, over the weekend, for military spending to be diverted from expensive high-tech defence projects to less glamorous counter insurgency kit is sensible.

But although scrutiny and debate over this engagement is healthy, the case for a withdrawal has not been made. The Western mission in Afghanistan, though overshadowed by the foolish invasion of Iraq and often poorly carried out these past eight years, remains a worthy one. Despite the crude characterisation of some opponents of the engagement, this is not a neo-colonial adventure. Nato troops, including Britain's contingent, are in Afghanistan at the invitation of the democratically elected government of President Hamid Karzai. And their purpose is to protect civilians from the depredations of the Taliban while the Afghan army builds up the capacity to take over the job.

They are also fighting for the protection of British citizens. Some three-quarters of UK terror plots under surveillance by the authorities have links to militants based on the Afghan/ Pakistan border. The Taliban granted al-Qa'ida a base before 2001. There is no reason to suppose they would not do the same again if they returned to power. Our own security is bound up with the safety of the Afghan people. There are encouraging signs that Nato's commanders have learned from past mistakes in Afghanistan. Its new military head has laid out a clear strategy based on pushing the Taliban back and holding territory once gained. He has also ended the counterproductive policy of destroying the opium crop and made it an explicit goal to minimise civilian casualties.

This new approach means there is still hope for this campaign. As Jerome Starkey's revealing report today demonstrates, there is little desire among ordinary Afghans for the return of the Taliban. And support for Nato troops in the country remains solid.

It will, however, be a long, hard and uncertain slog. This new military push is designed to increase security for next month's Afghan presidential elections. But it would be naïve to imagine that the Taliban will disappear after August. Experience tells us that they will return and that there will be further military casualties. We need to be mentally prepared for the duration of this vital mission to secure Afghanistan's democratic future, as well as the likely human cost.