The political half of America's strategy in Afghanistan is now in ruins. This is not just due to the debacle of the Afghan presidential elections. Eight years after US troops arrived in the country, as General Stanley McChrystal conceded in his report to President Barack Obama, there is a "crisis of confidence" among the Afghan people in their government.

As a result of the collapse of the political strategy, Washington's military mission now appears to have no goal beyond the avoidance of defeat. Asked to define victory, the US special envoy, Richard Holbrooke, could only say, "We'll know it when we see it." American and allied soldiers should not be asked to sacrifice their lives for such an unclear goal.

But the west should not simply leave. That would repeat the error of the 1990s when the US abandoned the region, contributing to the chaos that helped nurture the attacks of September 11 2001. The choice is not between scuttling away or deepening an open-ended military engagement. Neither is feasible.

The US and its allies need to recognise two facts and shape their strategy accordingly: successful "nation-building" in Afghanistan can only be undertaken by Afghanistan's own people; and, above all, it is the western military presence in Afghanistan that is driving support for the Taliban both there and in Pakistan. Put these together and what results is the need for a carefully phased exit strategy combined with a military and diplomatic strategy vis-a-vis the Taliban.

This will involve talking to the Taliban leadership. The Taliban today probably does not enjoy the support of a majority of Pashtuns -- but then, neither the IRA in Northern Ireland nor the FLN in Algeria were supported by a majority of their communities. To continue their fight indefinitely, such groups only need to be stronger than any other group in their community, and to appeal to one deeply felt idea. In the case of the Pashtuns of Afghanistan and Pakistan, that is a strong desire for the departure of western forces from Afghanistan. From this point of view, the notion that the western presence is protecting Pakistan from the Taliban misses the point completely.

The west should therefore pursue a political solution, open negotiations with the Taliban and offer a timetable for a phased withdrawal in return for a ceasefire. This should begin with the military pulling out of specific areas in return for Taliban guarantees not to attack western bases and Afghan authorities in those areas.

If the Taliban refuses such terms, then military pressure should continue. The point should not be to eliminate the Taliban -- which is impossible -- but to persuade it to agree to a deal. Similarly, a new approach to Pakistan should focus not on putting pressure on the Pakistani state to destroy the Afghan Taliban on its territory, but on persuading Islamabad to help bring the Taliban to the negotiating table. Meanwhile, Kabul should be secured as a neutral space by the establishment of a UN peacekeeping force from Muslim countries.

This approach should be combined with political reforms to decentralise the Afghan state and with a move from a presidential to a parliamentary form of government. In the parliamentary elections due next year political parties should be allowed to stand (at present this is banned). The Taliban should be encouraged to form a political party, which could take local power in many Pashtun areas through the political process and share in central government in Kabul. The west's central condition must be that the Taliban pledge not to permit sanctuaries for terrorism in areas it may dominate. Indications that the Taliban's alliance with al-Qaeda may be fraying need to be seriously tested.

Why should the Taliban agree to these terms if the west is leaving anyway? Because otherwise, after withdrawing ground forces, the US will give massive long-term military aid and air support to the anti-Taliban forces of non-Pashtun ethnicities, rekindling the civil wars of the 1990s, but on terms vastly disadvantageous to the Taliban and the Pashtuns.

This approach will not bring quick results. But the military-diplomatic strategy we have proposed offers a chance of a settlement and orderly withdrawal -- whereas the present strategy offers only endless quagmire.