Leading article: The time has come for some realism

Yesterday saw the 300th British fatality of the long Afghan war. It is, of course, an artificial milestone. As David Cameron pointed out, there is no reason why the 300th death of a British soldier in that conflict should be any more significant than the 299th or the 301st. But, rightly or wrongly, round numbers tend to capture the public's attention. This is a milestone, artificial or not.

The Prime Minister has called on the country to "reflect on the incredible service and sacrifice that the armed forces give on our behalf". And the public will certainly do so. But they will also, quite rightly, reflect on the nature of the mission itself.

Some truths about the UK's involvement in Afghanistan are now abundantly clear. First, the idea that Britain is able to extend an open-ended commitment to the country is dead. There is no popular support for that scale of commitment; nor are there the resources. The Army is already severely stretched on the ground. Troops lack body armour, helicopters and trained bomb disposal personnel. And with the defence budget facing a severe squeeze, things are most unlikely to improve. Second, the idea that we are masters of our own destiny in Afghanistan is a myth. The fig-leaf of the Nato command remains, but the reality is that this is a US-led operation.

It is a year since General Stanley McChrystal took command in the country. The results of his troop "surge" have been mixed. The Taliban have been driven from several southern strongholds by sheer volume of US and UK troops. But the operation to take the city of Kandahar, which has symbolic and strategic importance for the Taliban, has been delayed. And the Taliban's capacity to hurt foreign troops through roadside bombs remains undiminished.

In fairness to General McChrystal, he appears to understand the limitations of what can be achieved by military force alone in Afghanistan. His goal is to stabilise the country and build up the national security forces, rather than deliver a Western-style democracy. And he accepts that some form of political settlement with the more pragmatic elements of the Taliban is inevitable, indeed desirable.

To this extent, his efforts merit continued support. But the danger comes in deciding when enough is enough. Generals on the ground will always push for more resources and more time in the belief that a breakthrough is within reach. President Barack Obama has set a deadline of July 2011 for the start of the withdrawal of US combat troops from Afghanistan. But some Pentagon officials are already suggesting that longer might be needed. To break that deadline would be a mistake. In the end, political leaders must tell their generals when it is time to withdraw.

The great fear of any politician when it comes to disengaging from messy foreign interventions is the accusation that they asked soldiers to sacrifice their lives in vain. But the force of that charge depends on the goals politicians set and the rhetoric they use in justifying a conflict. The old argument that British and American troops in Helmand are protecting civilians in London and New York is not only incredible, it has made it much harder to pull troops out.

Mr Cameron and President Obama need to lower public expectations of what can be achieved in Afghanistan. They need to explain where in the region the true threat to the West's interests lies and what is the most practical and effective way of safeguarding those interests. These leaders are already being increasingly realistic in private on such matters. They need to share that same candour with their respective publics, rather then peddling the same old comforting fairy tales about the purpose of the Afghan mission. It is time, in short, for some realism.