A month ago, Afghan and American forces hosted a gathering of tribespeople near the Pakistani border that surpassed all expectations as an exercise in winning hearts and minds. More than 1,500 people attended. The Taleban were denounced by local leaders and dozens pledged to join the Afghan police. On Sunday, suicide bombers from the same region wrought carnage in Kabul's showpiece hotel. Eight guests and workers were killed and a government whose hold on power is already fragile was left to contemplate the arrival of al-Qaeda in what had been, by Afghan standards, a haven of relative calm.

The attack on the Serena Hotel was more than a sickening echo of pre-surge Baghdad. It was the first of its kind in the Afghan capital and was followed yesterday by Taleban warnings that more were imminent. Unlike Baghdad, Kabul has no green zone. If al-Qaeda has declared open season on the NGOs trying to rebuild Afghanistan, an exodus of the foreigners they employ would be one of the first consequences. Reconstruction would grind to a halt. Yet reconstruction is precisely what President Karzai and his myriad international partners must deliver to secure legitimacy. For all the talk in London and Washington of the need for a long-term presence in Afghanistan, time is critically short.

President Karzai's precarious position only adds to the urgency. Even in Kabul he has little say in security matters, for which responsibility still lies with the Nato International Security Assistance Force (Isaf). Nor is he in full control of his budget. Half of this consists of foreign aid, much of it earmarked for specific projects before it reaches Afghanistan. Ethnically, he is a lonely Pashtun in a government made up largely of Tajik veterans of the Northern Alliance. And his fear of being cast as a puppet by his rivals has led to a dangerous strategy of promoting inept officials motivated mainly by tribal loyalty and avarice.

A year ago, Mr Karzai dismayed Isaf commanders by sacking a new governor of Helmand province who had pledged to co-operate with Western efforts to curb heroin poppy production. He has since turned a blind eye to claims of involvement in drug trafficking by his half- brother and Interior Minister. But his political clumsiness was most evident last month in his peremptory expulsion of two Irish diplomats for holding covert negotiations with the Taleban.

The expulsions were meant as a signal of Mr Karzai's independence - of both Western advisers and the Islamists regrouping against him in Pakistan's tribal areas. But he would be unwise to seize on the Serena Hotel bombing as evidence that covert talks are doomed. They were originally his policy, and they remain a vital strand of a broader strategy of winning rank-and-file Taleban back to the democratic cause.

At the same time, reconstruction must mean something to the peasant farmers among whom extremists recruit. British Forces in particular have laboured heroically to make this happen. Others, including the Department for International Development, have realised late but not too late that their role, too, will involve real risk and sacrifice. If they can deliver concrete alternatives to the Taleban's medievalism, Mr Karzai may yet break the narco-dependency that comes with it. If not, the just war triggered by 9/11, and still being fought by 38 nations, could be lost.