Last weekend’s Nato summit in Chicago was mostly about how to get Nato troops out of Afghanistan without causing too much embarrassment to the western governments that sent them, and a little bit about how to ensure that the Taliban don’t take over again once the western troops leave.

The timetable for Nato’s withdrawal is now graven in stone: all western troops will be withdrawn from actual combat by the end of 2013, and they will all be out of the country by the end of 2014 (except the French, who will all leave by December of this year). This timetable will be adhered to no matter how the situation on the ground develops — or more likely, degrades — in the next two years. After that, it’s entirely in the Afghans’ hands.

There was some pretty rhetoric to soften this harsh fact: “As Afghans stand up, they will not stand alone,” declared President Barack Obama. But alone is exactly where they will be, although Nato is promising to send the Afghan government $4bn a year to enable its army to stand up to the Taliban. The western alliance has finally accepted that if the foreign troops cannot defeat the Taliban in 11 years, they are most unlikely to do so in 13 or 15 years.

The Russians could have told them that. “Our soldiers are not to blame,” Gen Sergei Akhromeyev told the Soviet Politburo in 1986. “They’ve fought incredibly bravely in adverse conditions. But to occupy towns and villages temporarily has little value in such a vast land where the insurgents can just disappear into the hills.”

So if Nato is now conceding that the Taliban cannot be crushed by military force, then why is it going to keep its troops in Afghanistan for another two-and-a-half years before acting on that conclusion? Some of them will die as a result of that decision, and quite a few Afghans will be killed because of it, too. Apart from temporarily saving the face of various western governments, what purpose will their deaths serve?

Nato’s argument is that another two years will leave the Afghan army in a better position to defend the US-installed government of Hamid Karzai after western troops leave, but there is absolutely no evidence that it is true. Indeed, of the 150-odd western troops killed in Afghanistan so far this year, twenty were killed by the Afghan troops that Nato is supposed to be training for this role.

The ‘Afghan National Army’ is not fit for purpose, and the outcome after Nato troops leave will probably be the same whether they all go home this year or stay until 2014. So what is that probable outcome?

Karzai may not fall immediately: the $4bn a year that Nato is promising to pay for the maintenance of his army will probably preserve the status quo for two or three years. But no more: it is most unlikely that the subsidy will be extended when it comes up for review in 2018.