Leading article: A war with high costs and doubtful benefits

When British troops joined US forces in Afghanistan almost seven years ago, there was little doubt, and even less controversy, about the mission. It was to capture Osama bin Laden, the presumed mastermind of the September 11 attacks on the US, destroy the camps where the 9/11 attackers had trained and prevent Afghanistan from ever sheltering terrorists again. The flight of the Taliban regime as the Northern Alliance advanced, and the talks from which Hamid Karzai emerged as President, seemed to hold the prospect of a brighter future after three decades of occupation and civil war.

The task of foreign forces was clear: it was to help Afghans rebuild their ravaged land and transform it into a law-governed state. It was a task of peace-keeping and reconstruction that enjoyed widespread international support. The notion that it might become a long-term combat operation was very far from anyone's mind. Last weekend, with the deaths of three British servicemen at the hands of a suicide bomber, the number of British casualties reached 100. The vast majority were killed in the past two years.

It is, of course, invidious to treat the 100th death as more significant than any other. Each, from the first to the 99th, has left grieving relatives and friends. But the day on which British casualties reached three figures inevitably prompts reflection, not only on the human cost of the Afghanistan mission to date but on the likely cost of our involvement into the future. This is not an operation whose end is anywhere in sight.

The expression "mission-creep" has been used, justifiably, of what has now become –for many British troops, at least – a war. But it is worth noting that until two years ago only seven British servicemen had died. The cost and benefits seemed proportionate; the undertaking seemed eminently worthwhile. What is more, foreign involvement was positively welcomed by many Afghans, in a way that it never was in Iraq.

Iraq, regrettably, explains much of what has gone wrong. For more than two years, the US and Britain were otherwise engaged. As the war in Iraq turned bad, manpower and hardware, along with political will, were all concentrated there. In Afghanistan meanwhile, the Taliban emerged from hiding, President Karzai's power contracted ever closer to the capital, Kabul, and the opium poppy crops multiplied.

In 2006, the US asked the British to try to wrest back control of Helmand province. What had begun five years before in the centre and north of the country as a high-minded, light-touch exercise in peace-keeping and rebuilding has now degenerated into messy warfare in one of the most lawless regions of the world. Just holding the line against the resurgent Taliban is increasingly regarded as success. Britain now has many more troops stationed in Afghanistan than in Iraq. Military commanders describe Helmand as more dangerous than Basra and Baghdad ever were, and even as they acknowledge that the Taliban cannot be beaten by military means alone, it is military means they must increasingly apply. And the more the dangers outstrip the prospects of success, the less other countries want to risk their troops. Afghanistan is straining the unity of Nato even more that its disputed operation in Kosovo.

Now is not the time to end our commitment to Afghanistan. But it is a time to review what has been achieved, and what can realistically be achieved in the months to come. Commenting on the 100th military death yesterday, the Prime Minister said this of British casualties in Afghanistan: "They have paid the ultimate price, but they have achieved something of lasting value." We wish that we could share his confidence.