Leading article: The dangerous hint of 'mission creep'

In principle, we do not take issue with the deployment of British troops in Afghanistan. The US-led intervention, which led to the overthrow of the Taliban, enjoyed genuine international support. Its prime purpose was to root out al-Qa'ida; regime-change was a by-product of this, and one in which Afghans themselves took the lead.

That relatively few of the hopes harboured for the government of Hamid Karzai have been realised reflects in part the difficulty of bringing peace to Afghanistan after so many decades of volatility. But it also reflects the dissipation of US and British interest once the adventure in Iraq had been launched. Troops, strategic planning and media focus were diverted to Iraq. Urgent pleas from President Karzai for more assistance went unanswered.

The folly of this course has been apparent for some time. As American and, to a lesser extent, British forces have struggled to turn back the tide of violence in Iraq, the situation in Afghanistan has been deteriorating. Regional warlords have strengthened their hold outside Kabul and there have been signs that the Taliban could be on the way back.

When the Government announced that it was to deploy 3,000 British troops to Helmand province, it said the purpose was peace-keeping. We pictured kindly British soldiers in berets keeping order and helping with civilian reconstruction projects, such as well-drilling. A more contentious aspect of their job was to redouble previous - failed - efforts to eradicate the cultivation of opium poppies.

Now, however, the Defence Secretary, John Reid, has thrown into the equation, almost casually, the small additional matter of fighting terrorists. This is something specialist US troops have been engaged in continuously since the intervention. It is a dangerous and thankless task in forbidding terrain and not the sort of mission that we understood the new British contingent would be engaged in.

Mr Reid has tried to square the circle by explaining that, although the British mission was mainly about reconstruction, "terrorists will want to destroy the economy... and the government that we are helping to build up". But there is a difference between defending rebuilt infrastructure against hostile raids on the one hand and setting out to track down "terrorists" on the other. Once the line is blurred, accusations of "mission creep" are inevitable.

If the Government wants to keep public opinion on its side over Afghanistan - as it needs to, given the debacle in Iraq - it must be honest about why the troops are there. Is peace-keeping their main role, or will they be supplementing US search-and-destroy operations in the mountains of the south-east? This is a justified suspicion that Mr Reid needs to allay.