During a recent visit to the Gulf state of Oman a senior American officer was asked: "What's plan B?" With a wry smile he responded: "What's plan A?"

Yesterday's capture of Mazar-i-Sharif by the Northern Alliance was the first piece of good news for the US and Britain almost since the "global war on terrorism" began, and the Pentagon, understandably, made much of it.

But even this is not without its problems. Mazar has fallen far later than expected, and it will be some time before its prized military airfield can be used by the American-led coalition. And the Alliance taking the city was not what was wanted at all in the first place.

The scenario initially envisaged by the Bush administration was that prolonged bombing would make the Taliban "recant" over their refusal to hand over Osama bin Laden. If they did not, there would be a series of commando raids by special forces who would also organise insurrections in the Taliban's Pashtun heartland. The result would be the swift downfall of Mullah Mohammed Omar's regime.

The Northern Alliance was not seen as part of the winning equation, mainly due to pressure from Pakistan which feared the Russian, central Asian, Iranian and Indian influence that the Alliance represented. Pakistan tried to persuade Washington that any replacement government should have a large-scale Pashtun presence, preferably of "moderate" Taliban.

But the Bush administration's scheme unravelled swiftly. Mr bin Laden was not handed over, the Taliban regime did not collapse, and Pakistani-sponsored talks in Islamabad to form the broad-based coalition of the future got nowhere as the Northern Alliance kept away.

The much-vaunted commando operations got off to a disastrous start. The one and only raid into Afghanistan, by US Rangers, was a purely cosmetic exercise for the benefit of the television cameras on targets which were meant to be poorly defended. But the intelligence was faulty and the ferocity of the Taliban counter attack made the Americans beat a hasty retreat, one of the helicopters losing its undercarriage as it attempted to get away.

The Pentagon's planners were forced to rethink. Within 24 hours they had asked for special forces from Britain and Australia, and there were no more raids by American forces.

Military help was given, belatedly, to the Northern Alliance. US warplanes began carpet bombing the Taliban front line facing the Alliance positions and the opposition fighters were urged to advance.

There has been no repetition from London and Washington of statements about the Northern Alliance's human-rights record which, just a few weeks ago, appeared to pronounce it unfit to be considered as a partner of the West or a leading participant in the future Afghan government.