Leading article: Any intervention must be based on international law

The uprising against Muammar Gaddafi's four-decade long rule in Libya has already proved far bloodier and more costly than the popular revolts in either Tunisia or Egypt. Marshalling his land, sea and air forces, and contingents of mercenaries to boot, Gaddafi has thrown all his considerable resources into retaining power. As of yesterday, his fightback appeared to have stalled at the oil city of Brega. But if he prevails there, only one major settlement remains between him and the opposition's headquarters at Benghazi.

With time for the opposition seemingly running out, the Libyan leader's brutal singleness of purpose has left the outside world flailing for a response. All natural sympathy rests with the opposition forces; the tide of history is surely with them. The question is whether to assist them, and if so, how.

At the weekend, the Arab League agreed to ask the United Nations Security Council to impose a no-fly zone over Libya. Only Syria and Algeria dissented. Britain and France also support the idea. Yesterday, one of Britain's foreign policy heavyweights, Sir Malcolm Rifkind, called on the West to supply the anti-Gaddafi forces with arms. His appeal came as foreign ministers from the Group of Eight industrialised countries met in Paris, with Libya at the top of their agenda. A plan for a no-fly zone could be presented to a Nato meeting today, with a formal resolution to be submitted to the Security Council after that.

There are many who will see such a timetable as unduly ponderous, given the speed with which Gaddafi has mounted his counter-attack. Yet if any lesson has been learnt from Iraq, it should be that effective action cannot be mounted without the widest possible support, including from within the region, and that it should be legal – which means it should have, if at all possible, a UN Security Council mandate. The rights and wrongs of what is happening in Libya may be clear, but the politics and the practicalities are not as straightforward as those clamouring for immediate intervention suggest.

While the governments of Britain and France support a no-fly zone, they have not so far been able to convince all their fellow Europeans. Crucially, the US administration has also held aloof. In Nato, Germany and Italy are cool about a no-fly zone, while Turkey is strongly opposed. The support of the Arab League is a positive development, which proceeds from its position that Gaddafi has lost legitimacy, but there is no guarantee that Russia and China will be swayed.

The Prime Minister yesterday described a no-fly zone as "perfectly deliverable", but mustering the forces required could take weeks, and – as the US Defence Secretary, Robert Gates, has pointed out – controlling the skies over Libya is a far more demanding proposition than protecting Bosnia or sectors of Iraq. Whether it is necessary, as Mr Gates has also argued, to start with air strikes is contested, but the risks cannot be underestimated. Gaddafi has demonstrated that he has air power and is prepared to use it. The enforcement of a no-fly zone could rapidly escalate into intervention in some form on the ground.

To these risks has to be added the uncertainty that persists across the region. The arrival of Gulf troops, mainly from Saudi Arabia, in Bahrain yesterday potentially creates a new source of instability. The situation in Egypt remains volatile. Above all, it must be asked whether it is wise for Western countries to assist an uprising in Libya that cannot sustain itself; that way lie accusations of neo-imperialist ambition and the prospect of messy defeat. There can be no substitute for an international consensus. Circumspection and the law must rule the day.