As the contours of an American military response to last week's attacks loom into view, the enemy remains elusive. One target is obvious: the Taliban rulers of Afghanistan. If they refuse to hand over Osama bin Laden and his terrorist supporters, the mullahs will provide the US with every justification for a military strike. But what kind of strike? And with what precise objective?

Given the nature of Afghan society and topography, it is easier to say what the objective should not be. The list of outside countries that have tried, and failed, to invade and occupy Afghanistan is long enough to suggest that another attempt should not be contemplated.

But it is even harder to define a more precise or limited aim - partly because of the country's desolate recent history and partly because of the extraordinary role that Mr bin Laden appears to play in its present.

Afghanistan's recent history is a tale of violent tribal anarchy, inflamed by the presence of thousands of Islamic warriors trained and sent by Arab nations, with the support of the US, to oppose the Soviet army after 1985. The Taliban, with its brutal vision of Islamic society, emerged as a force of order to fill this vacuum. Many war-weary Afghan citizens accepted it because they knew the alternative was worse.

As to Mr bin Laden, it is hard to distinguish his role from the Taliban. He seems to bankroll what infrastructure the country possesses; some believe he is de facto Afghanistan's defence minister. President George W. Bush was correct in vowing to make no distinction between terrorists and the governments harbouring them: in attacking Mr bin Laden, he will by definition be attacking the Afghan government.

To this objection, the only conceivable reaction is: so be it. It should be possible to overthrow the Taliban by judicious use of special forces and air power. Even before last week's attack, it was among the most reviled governments on earth, including in the Islamic world. It is utterly dependent on fuel and food supplied via Pakistan which is, for now, siding with the US against it. At home, it is not in full control of its territory and is opposed by significant numbers of its countrymen - although the only half-credible opposition leader was murdered two days before last week's attacks.

The US and its allies can thus reasonably hope to rid the world of Taliban rule in Afghanistan. Capturing or eliminating Mr bin Laden and his supporters will be much harder. In the meantime, Afghanistan will have reverted to chaos. The choice may therefore be between a long-term presence in the country, with all the attendant risks, and a short-term attack that undermines the regime and should reduce the terrorist danger. The latter seems the lesser evil.