Flames billow up from the hulks of eight Libyan navy vessels destroyed by Nato air attacks as they lay in ports along the Libyan coast. Their destruction shows how Colonel Muammar Gaddafi is being squeezed militarily, but also the degree to which the US, France and Britain, and not the Libyan rebels, are now the main players in the struggle for power in Libya.

Probably Gaddafi will ultimately go down because he is too weak to withstand the forces arrayed against him. Failure to end his regime would be too humiliating and politically damaging for Nato after 2,700 air strikes. But, as with the capture of Baghdad in 2003, the fall of the regime may usher in a new round of a long-running Libyan crisis that continues for years to come.

It has all developed rather differently from what the French and British appear to have imagined when they first intervened in March to save the citizens of Benghazi from Gaddafi's advancing tanks. If this was their sole aim, the air strikes were successful. The roadside from Benghazi to Ajdabiya is still littered with the carcasses of burned-out armoured vehicles. But months after William Hague was suggesting that Gaddafi was already en route for Venezuela he is still in Tripoli.

Three months after the start of the Libyan uprising Gaddafi's troops have failed to capture Misrata, but the rebels do not look capable of advancing towards Tripoli. They have broken the siege of Misrata partly because their militiamen now clutch hand radios and can call in Nato air strikes. This close air support is effective and is along the lines of the tactical air support given by the US to the Northern Alliance soldiers in Afghanistan in 2001 and the Kurdish peshmerga fighters in northern Iraq two years later.

The Libyan government and opposition forces are both weak. The fighting forces that have been clashing on the desert road between Brega and Ajdabiya, south of Benghazi, often number no more than a few hundred half-trained fighters. Gaddafi's troops, with which he tries to control this vast country, number only 10,000 to 15,000. This is not always obvious to anybody who is not an eyewitness because the foreign media on the spot is bashful about mentioning that there are sometimes more journalists than fighters at the front.

One dispiriting outcome of the Libyan uprising is that the future of Libya is decreasingly likely to be determined by Libyans. Foreign intervention is turning into an old-style imperial venture. Much the same thing happened in Iraq in 2003 and in Afghanistan in the past few years. In Iraq, the US invasion to overthrow Saddam Hussein, a ruler detested by most Iraqis, soon turned into what many Iraqis saw as a foreign occupation.

As in Iraq and Afghanistan, the weakness of France and Britain is their lack of a local partner who is as powerful and representative as they pretend. In the rebel capital Benghazi there is little sign of the leaders of the transitional national council, which is scarcely surprising, because so much of their time is spent in Paris and London. In Washington, the White House was a little more cautious last week when Mahmoud Jibril, the interim Libyan prime minister, and other council members came to bolster their credibility and hopefully get some financial support. More circumspectly, the Libyan rebel leaders were there to allay American suspicions that the Libyan opposition is not quite as cuddly as it claims and includes al-Qa'ida sympathisers waiting their chance to seize power.

The Libyan opposition may be weak but is not quite so naive or inexperienced as it sometimes appears. Its leaders are quick to play down eastern Libya's tradition of militant Islam. In the town of Al Bayda, on the long road from the Egyptian frontier to Benghazi, I saw a large notice in French addressed to any passing foreigners, denying any link with al-Qa'ida. This is largely but not entirely true. One Libyan observer in Benghazi explained: "The only people in this part of the country who have any recent military experience are those who were fighting the Americans in Afghanistan, so of course we send them to the front."

Wars often widen and deepen existing fissures in a society. The rebel transitional national council likes to play down suggestions that it is primarily a movement from Cyrenaica, the great bulge of eastern Libya where Gaddafi has always been unpopular. But he has held on to most of western Libya. Today these two halves of Libya, separated by hundreds of miles of desert, increasingly feel like separate countries.

Libyans on the ground have fewer inhibitions about discussing these differences. Outside some beach huts in Benghazi used to house refugees, I spoke to oil workers from the oil port of Brega, a town of about 4,000, who had fled when Gaddafi's forces captured it. A manager from the gas fields said: "Gaddafi's people got hold of a book with all our names because they wanted to see who came from east Libya and in their eyes would naturally be a rebel."

Of course, Gaddafi's opponents don't just come from the east. It is fair to assume that most Libyans from all parts of the country want him to go. He clings on because he rules through his family, clan, tribe and allied tribes, combined with his ebbing control of the ramshackle Libyan government and military machine. Everything within the part of Libya he still controls depends on Gaddafi personally. Once he goes there will be a political vacuum that the opposition will scarcely be able to fill.

Could the war be ended earlier by negotiation? Here, again, the problem is the weakness of the organised opposition. If they have the backing of enhanced Nato military involvement they can take power. Without it, they can't. They therefore have every incentive to demand that Gaddafi goes as a precondition for a ceasefire and negotiations. Since only Gaddafi can deliver a ceasefire and meaningful talks, this means the war will be fought to a finish. The departure of Gaddafi should be the aim of negotiations not their starting point.

One surprising aspect of the conflict so far is that there has not been a greater effort to involve Algeria and Egypt, the two most powerful states in North Africa. This would make the departure of Gaddafi easier to negotiate and would make the whole Libyan adventure look less like West European imperialism reborn. The aim of Nato intervention was supposedly to limit civilian casualties, but its leaders have blundered into a political strategy that makes a prolonged conflict and heavy civilian loss of life inevitable.