Patrick Cockburn: To have an impact, this kind of intervention needs clear objectives

Western nations will soon be engaged in a war in Libya with the noble aim of protecting civilians. But the course of such a conflict is impossible to predict. The UN Security Council has authorised the use of "all necessary measures", including aerial bombing, to avert a military victory by Colonel Gaddafi. It was not at all clear what this authorisation will mean in practice.

The Americans, the British and the French have come to understand that establishing a no-fly zone is not enough. Colonel Gaddafi's main strike force consists of tanks and infantry, so inability to use aircraft might not be sufficient to stop him capturing Benghazi and eastern Libya. Given that most of the Libyan population lives in cities and towns close to the sea, air strikes on the main coast road might stop the regime's motley forces. But it is the tradition of wars in the Middle East that the first days of foreign involvement are always the best.

In Afghanistan and Iraq, there was clarity of objective – which was to overthrow the Taliban and Saddam Hussein, respectively.

There is less clarity with Libya. Is the aim to defend the rebels in the eastin the east of the country?

Will it extend to any surviving rebel strongholds in the west, such as Misurata, where there has been street fighting? Is the aim to get rid of Colonel Gaddafi? No-fly zones on their own are difficult to make work effectively. They may have an intimidatory effect, but this depends largely on the implied threat of air strikes. A no-fly zone alone would not have saved the Shia or Kurdish uprisings of 1991, because Saddam Hussein had armour and mechanised divisions, which militiamen could not resist.

In 1996, Saddam Hussein captured the Kurdish capital, Arbil, with the co-operation of one of the Kurdish parties and the US did not intervene. The occasions when outside air power does work is when strike aircraft are being directed by specialist teams of foreign soldiers on the ground, acting in co-operation with local militias.This caused the collapse of the Taliban in 2001 and of the Iraqi government forces in northern Iraq in 2003. The problem is that it is not clear who the US and Europe will be aiding. The most surprising development in this uprising is that it began with the defection of military units but these, until the last few days, have not appeared on the battlefield. Hillary Clinton says what really changed her mind about intervening in Libya was the Arab League's statement calling for action. But the members of this somewhat discredited body are mostly autocracies which may dislike Gaddafi, but whose methods of government are no less repressive.