The faster we act, the sooner we can leave

The decision to deploy thousands of American, British and other allied troops in Afghanistan was inevitable after the events of the past week. The dramatic eclipse of the Taliban in northern Afghanistan created a power vacuum that has to be filled. As before in this crisis, the British Prime Minister has been ahead of the Americans in urging this. Allied troops would have been on their way before now, had not Washington taken some days to make its own judgements, amid Tony Blair's telephone urgings.

The risk is very real, that Western troops will become entangled in a bloody civil war. But, having launched military action in Afghanistan, America and its allies are committed to saving the country from itself. By putting troops on the ground - and before long they may need a great many - they have a chance of securing the Taliban's defeat before winter; searching for Osama bin Laden on much more favourable terms; and averting a bloodbath, as rival Afghan warlords strive for local power and revenge. Today the consequences of failing to act fast look far worse than the perils.

Military experts point out that the "war" of recent weeks has not been a war in the sense Western soldiers understand it, with ground battles between serious armies. Afghanistan is not a nation state. It is a geographical grouping of rival tribes, whose loyalties are overwhelmingly tribal rather than national. The Northern Alliance has not had to do much real fighting. American air power has pounded the Taliban fighters and their supply lines until they withdrew, leaving the Alliance to occupy abandoned territory, and mop up stragglers.

This process may well continue until all the Taliban strongholds have fallen, and the mullahs and their fighters have retreated into the hills. No one can guess whether it will take a week or six months. But an outcome cannot be achieved by air power alone. Ground troops have to occupy the territory, to fill the vacuum, to create sufficient order at least to allow humanitarian aid to get through.

Until now, Washington has chosen to let the Northern Alliance take the lead on the ground, with advice and support from American and British special forces operating from secure bases they have established in Alliance territory. But the limits of this option have been reached. First, there are not enough Alliance fighters to take over the rest of the country. Second, the deeper they push into Pashtun territory in southern Afghanistan, the smaller the likelihood that northern tribesmen can control the alien ground they occupy, least of all humanely.

That is why American and British troops - with at least token contingents from other nations, especially Muslim ones - must go in. Even if they do not launch offensive operations against the Taliban, they will maintain some kind of order until the United Nations can take over. It will be surprising if they do not find themselves fighting somebody. Even if the Taliban is pushed back from Afghan cities, continued guerilla war is inevitable. But unless the Coalition speedily grips the situation on the ground, all the gains of the past week could evaporate. The Americans must keep bombing wherever Taliban forces show signs of rallying.

It is too late to agonise about the dangers of getting our troops involved in a dangerous entanglement. Washington must see this thing through to some sort of successful outcome, which can be achieved only by committing a substantial, disciplined foreign force to hold the ring. It is dismaying that the United Nations still seems so far off being ready to assume responsibility for creating an Afghan administration. At all costs, the West must not be seen to be imposing a puppet regime. Experts on Afghanistan are deeply sceptical about promises of a "broadly-based coalition government", though it is a phrase the Prime Minister has used repeatedly. Over centuries, Afghans have shown themselves chronically lacking a national, as distinct from tribal community of interest. Some de facto system of partitioned administration seems far more likely to work, even if there is a nominal national council headed by the elderly king, as the allies presently seem to favour.

The men and women in Washington and London responsible for the longterm struggle against international terrorism are acutely sensitive to the danger of getting bogged down in Afghanistan, when al Qaeda's tentacles reach into many other countries. But they argue that success in the northern fastnesses will be a vital first step. There is a tiny glow of optimism - impossible to judge how realistic - that Osama bin Laden could be cornered sooner rather than later. This would be a huge boost for the Coalition's prestige.

Britain is now confident that America can be dissuaded from any alarming military "mission creep" beyond Afghanistan. It believes Washington will not seek to undertake any big new military action against Iraq or other rogue state unless directly provoked. The struggle against terrorism will be pursued by less dramatic means. There cannot be a single grand, overarching plan. The campaign will have to be waged in a long series of incremental steps embracing many nations and many years. After Afghanistan military action will play a marginal, rather than central role.

But the first incremental step has got to begin in Kabul. By any measure, the crumbling of the Taliban this week is good news. The deployment of troops poses many problems for Washington and its allies, but they are a lot less daunting than the danger openly discussed a fortnight ago, that Washington would have to launch a major land war in Afghanistan next spring. The sooner an international force can be established on the ground to dominate the rubble left by the Taliban, with luck the sooner the British and Americans can hand over to the United Nations and get out of Afghanistan again.