Leading article: Time for Nato to fill in the blanks on Afghanistan

As if one insoluble problem were not enough, world leaders moved directly from discussing the woes of the eurozone at Camp David at the start of the weekend, to the equally intractable challenges of the Nato summit in Chicago at the end of it. Even without the thousands of protesters lining the streets, the meeting would not have been an easy one. And the issue at the top of the agenda is the most difficult of all: the withdrawal from Afghanistan.

There is certainly much that needs to be decided by the time the conference concludes later today. So far, although the 2014 deadline for pulling all combat troops out of Afghanistan was agreed two years ago, few further details have been fixed. Now they must start to be. For Chicago to be a success, there must be clear progress in establishing both the sequence of troop withdrawal and the massive logistical programme needed to bring out the estimated £40bn-worth of matériel in the region.

Trickier still, agreement must be reached on Nato's future role in the country. As the departure date draws closer – and the security situation in Afghanistan shows little sign of unambiguous improvement – it is more difficult than ever to claim it is more than a war-weary concession to the unconquerable Afghan terrain and an abandonment of its people to a highly uncertain, and potentially violent, future. Indeed, President Obama's meeting with his opposite number, Hamid Karzai, yesterday, to discuss a possible political settlement with the Taliban, only underlines the degree to which the goalposts have, once again, been shifted.

In order for the formal end of the decade-long conflict not to pitch Afghanistan straight back to its former position as a human rights black spot and safe haven for international terrorists, Nato proposes to retain an extensive training and advisory role even after combat troops have left. Not only must the scope of such an endeavour now be decided, so must its details. And central to that is the vexed question of money.

Efforts are already being made to ensure the Afghan National Security Forces can reliably take over from their international counterparts after 2014. But the price tag for ongoing support is $3bn per year, of which the US will fund around half. Britain has already committed to an annual £70m. Similar promises must be wrung out of the remaining Nato leaders in Chicago. But with many still feeling the pinch from the global financial crisis, and scaling back their domestic defence spending as a result, the case is harder to make than ever – and the proximity to the G8 meeting on the euro crisis will hardly help.

Neither is the politics any easier. Indeed, with so many domestic electorates having long lost their stomachs for the war, the international alliance is shakier than ever. In April, the Australian Prime Minister announced plans to withdraw all troops a year early, by the end of 2013. And François Hollande, France's new President, pledged in his election campaign to withdraw French forces by the end of this year. There is some room for manoeuvre – Mr Hollande, for example, has said France will continue other forms of support – but the risk is that a drip-drip of departures will leave only a US-led rump (including Britain) that is more exposed, less effective, and at greater risk. Unless the Nato summit can ensure that the trickle of departures does not become a flood, then the case for an accelerated, comprehensive withdrawal only becomes stronger.

There are other issues on the table in Chicago, not least the missile defence programme that ruffles feathers in Moscow. But it is with regard to Afghanistan that there are questions that can no longer wait for answers. Amid all the uncertainties, however, one thing is clear: even when the war there is officially over, our involvement will not be.