Leading article: This mission remains far from accomplished

It was a neat parallel to draw, but not one that bore close examination. On an unexpected stopover in Afghanistan yesterday, the Prime Minister told British troops at Camp Bastion in Helmand province that they were showing exactly the same courage, professionalism and dedication as the country's winning athletes in Beijing, and that everyone would remember them for it.

There can be no doubt about the courage, professionalism and dedication of the troops serving in Afghanistan. The difference lies in the context, and a pretty big difference it is, too.

For while Great Britain's athletes will be returning home in a few days, wreathed in medals, their task more than accomplished, the British mission in Afghanistan continues. And not only is no end in sight, but any prospect of a satisfactory conclusion seems to be receding by the week. When Mr Brown cited commanders as saying that substantial progress was being made against the Taliban, he declined to mention what veritable mountains there remained to climb.

Regrettably, the commitment to Afghanistan's future made by Western leaders and Nato almost seven years ago seems to have produced little in the way of lasting advances. It is hard to escape the impression that the overall situation, in terms of security at least, has been going backwards. And the nature of the international commitment has changed out of all recognition.

What began as a benevolent mission focused on peacekeeping and reconstruction, assisting Afghans to rebuild their ravaged country, has become a major military operation whose central purpose – beyond maintaining Hamid Karzai in power in Kabul – is increasingly unclear. And where it is clear, as in Helmand province where British troops are fighting to keep the Taliban at bay, the purpose seems increasingly at odds with the mood of the local population. Those who came as peacekeepers are fast becoming hostile occupiers in Afghan eyes.

Some argue that the growing number of British and other casualties – most recently the 10 French troops killed and more than 20 injured in an ambush only 50km from Kabul – reflects desperation on the part of Taliban forces as they fight to hang on to shrinking local power bases. But the maps of military activity tell a different story. They indicate that the Taliban are strengthening their hold on rural areas in the south and east and are moving ever closer towards Kabul, in a pattern familiar from every change of central power in Afghanistan in the past half-century.

It is high time that the advances of the Taliban, rather than being denied, were treated as an alarm call. If the now-fractured international alliance that went into Afghanistan so optimistically can agree on anything, it is surely that the return of the Taliban must be resisted. Taliban rule kept Afghans in thrall, oppressed women and outlawed anyone who thought differently. Its failed state allowed al-Qa'ida to flourish. The Taliban's chief attraction now may be less its ideology than the security it manages to enforce, but that is also a measure of how far Nato has fallen short.

The latest word is that the US favours an Iraq-style "surge" in a last effort to stop the Taliban. This would require many more troops than the Europeans, Britain included, have hitherto been prepared – or able – to supply. There is also a risk that such an offensive would rebound. But Nato should be considering: if not a "surge", then what? We made a promise to the people of Afghanistan in 2001 that the West would not let them down again. It is a promise we are duty-bound to honour.