Our report today, that the Afghan National Police is unfit to take over responsibility for security, might be held to run counter to this newspaper's call for British troops to withdraw from Afghanistan. It might be argued that US and British soldiers should stay longer, to give the Afghan forces time to overcome the problems that have reduced British officers to "despair", and to build up the local police to the point where they can safely ensure law and order. On the contrary, we believe that our disclosure of classified Foreign Office documents strengthens the case that
The Independent on Sunday made last year: that it is time to plan the phased pull-out of British troops.

Our troops have been there for nine years now, on a mission that was justified, but the aims of which have become confused and open-ended. In those nine years, international forces have failed to fulfil the unrealistic promises to build up Afghan capability at anything like the speed that was envisaged. This newspaper has always drawn a sharp distinction between the British role in Iraq and Afghanistan. Indeed, the Iraq invasion was a terrible distraction from the task of rebuilding Afghanistan. But our report today suggests that Mr Brown continues to overclaim the progress that has been made, and to underestimate the difficulties that stand in the way of realising his rhetorical ambitions. The internal government documents admit that it "will take many years" to create an effective police force. The existing force is riddled with corruption and drug abuse, and non-existent "ghost recruits" may account for up to a quarter of its purported numbers.

If British and American troops continue to substitute for Afghan security forces, they will be drawn too far into the task of nation-building. Which might be fine, if poverty and the weakness of the institutions of civil society were Afghanistan's only problems. But in a country such as Afghanistan the mere presence of foreign troops on its soil is part of the problem, and a part that will get worse over time. The longer our troops are there, the more they act as a target for Islamist nationalism. Our judgement is that we have reached the point where the good that our military personnel do – and they do a lot of good work – is outweighed by this strategic consideration.

The ultimate test of our presence in Afghanistan must be whether it helps or makes worse the threat of terrorism on the streets of British cities. That is not the only test, of course, because a secondary motive in intervening there was the humanitarian one of hoping to make life better for the Afghan people. That altruistic concern was, again, entwined with self-interest, because prosperous and secure countries are less likely to sustain dictatorships or to harbour terrorists. (Although the experience of Pakistan should act as a warning against too simplistic a read-across.)

It ought to be clear, however, that we need to scale back our ambitions for our mission in Afghanistan. As Fawaz Gerges, professor of Middle Eastern politics at the London School of Economics, argued in an interview last week, al-Qa'ida is a "tiny fraction of what it used to be" – despite the fillip to its recruitment provided by the Iraq war. The threat of jihadist extremism now comes almost as much from the Pakistani Taliban and from similar organisations in Somalia and Yemen. The Afghan Taliban, on the other hand, is, in Professor Gerges's view, a primarily nationalist movement.

That is why we argue for a scaling back – and reorientation – of our presence in Afghanistan. We should aim to contain the Taliban rather than defeat it, and to monitor the border with Pakistan, using intelligence, special forces and civilian support rather than regular combat troops.

President Barack Obama's troop surge provides an opportunity for a staged British and then American withdrawal. Already, as we have reported, British troops have been pulled back from the most dangerous positions in Helmand province as American reinforcements have arrived. The surge at least provides the space for the Afghan army and police force, however inadequate, to step up; after that, we should make clear, security will be the responsibility of the Afghan government.

Thus Afghans can take control of their own destiny and we can raise our eyes to the wider horizon, and devote limited resources in proportion to the many other more pressing threats to our own security.