Only an all-out war can depose Assad. Anything less is like being 'half-pregnant'

Syria is close to following Afghanistan, Iraq and Libya as the target of a major Western military intervention. It certainly looks that way after the American decision last week to send weapons to the rebels in a move that can only deepen the conflict.

The supposed aim of the United States arms supply is to "tip the balance" in favour of the insurgents and force Bashar al-Assad's government to negotiate its departure from power. But Assad holds all but one of Syria's cities and large towns, so, to transform the military situation on the ground the US, Britain and France would have to become the main fighting force of the rebels and engage in a full-scale war.

Such a war would be similar to what happened in Afghanistan in 2001 when the cutting edge of the anti-Taliban offensive was strategic and tactical American air support. The anti-Taliban militiamen led by the Tajik, Uzbek and Hazara warlords of the Northern Alliance were few in number – I kept running into the same small units on the road from Kabul to Kandahar – and acted essentially as a mopping-up force that did little real fighting.

In northern Iraq in 2003 the Kurdish Peshmerga were careful not to advance anywhere until the Iraqi army facing them had been pummelled by US bombers. One Kurdish commander told me that his men could not advance an inch without US permission because "we have to co-operate with American air support". Much the same happened in Libya in 2011 when, for all the laudatory media coverage of the rebels, they would not have survived for more than a few days without Nato Special Forces on the ground and air power overhead.

Of course, the Western intervention in Libya started off with the declared humanitarian purpose of preventing Gaddafi capturing Benghazi and massacring its people. The reality was that Nato leaders were determined to overthrow the regime. The main role of Libyan militiamen on the road south of Benghazi was to appear on foreign television. One of the more amusing sights at the time was to watch cameramen asking other members of the media to stand to one side so viewers would not see that journalists were more numerous than Libyan fighters in the front line.

The message of these three wars is that if the US and its Western allies do intervene in Syria it is they who will be the main players while the rebels will have only bit-parts or be there to give a Syrian gloss to foreign intervention. There are already signs of this happening. Brigadier Salim Idris, the chief of staff of the Supreme Military Council of the Free Syrian Army (FSA), appears to spend more of his time giving interviews to foreign journalists than commanding his troops. Not that these are numerous since even his own aides admit privately that he can give orders to a maximum of 10 per cent of the insurgent brigades in Syria.

There is a disconnect between the rebels as they really are and as presented by Western politicians such as David Cameron. Suddenly there is international concern about what will happen in Aleppo if the Assad forces launch a counter-attack to drive the rebels out of the parts of the city they hold. The rhetoric is similar to that used by then president Nicolas Sarkozy and David Cameron over the need to save the people of Benghazi from massacre in 2011. It is a measure of British and French cynicism that they hardly seemed to notice when, 10 days ago, militiamen in Benghazi, whom they formerly supported, shot dead 31 Libyans protesting against militia rule.

Britain and France speak as if the struggle was between an overwhelmingly popular insurgency and a hated dictatorship. But it was a rebel commander, Abu Ahmed, in the al-Tawheed Brigade that is part of the FSA in Aleppo, who volunteered to a reporter earlier this year that 70 per cent of people in Aleppo support Assad. "They don't have a revolutionary mindset," the rebel officer lamented, blaming this on the FSA's oppression, and corruption caused by "parasites" who had infiltrated its ranks. Inured to horrors though Syrians have become, they were appalled last week to see pictures on Twitter of the mangled head of a 14-year-old boy selling coffee in the street in Aleppo. He had been shot twice in the face by rebels after they accused him of speaking ill of the Prophet. Also last week, rebels massacred 60 people in a Shia village in Deir Ezzor province in the east of the country.

The volume of propaganda justifying Western military intervention in Syria is so high because leaders advocating it know that polls show that such intervention is highly unpopular at home. Hence the White House's claim that it decided to arm the rebels when it finally became convinced that the Assad regime had crossed a red line by using chemical weapons including sarin gas. Anthony Cordesman of the Centre for Strategic Studies in Washington, while arguing for full-scale US intervention in Syria, says "the 'discovery' that Syria used chemical weapons may well be a political ploy. It seems very likely that the administration has had virtually all the evidence for weeks, if not months."

In fact, the evidence smells very like that for Saddam Hussein's weapons of mass destruction in 2003. It begs the question of why Assad should use small quantities of sarin gas knowing it would be used to justify Western military intervention when his forces are not short of artillery and every other weapon if they want to kill people. One curious aspect of the sarin gas story is that, at the end of May, the Turkish security forces said they had arrested in Turkey militants of the Syrian rebel al-Nusra Front, affiliated to al-Qa'ida, who had in their possession a 2kg cylinder filled with sarin. This is far more substantial evidence for the possession of poison gas than anything alleged against Assad's forces, but the US, Britain and France showed no interest.

At the G8 meeting in Enniskillen tomorrow it may become clearer how far the US and its allies distinguish between propaganda and reality in relation to Syria. Do Britain and France really imagine that a mix of bluff, threats and a limited supply of infantry weapons will have a decisive impact on the battlefield? Cordesman argues for a no-fly zone that should be rapidly transformed into "a de facto no-move zone". This is the most effective way to allow the rebels to defeat Assad if they can. In other words, only an all-out war by the West will work against Assad; anything else will be like being "half-pregnant".

Cordesman is probably right in his military assessment but surely wrong, as we have seen in Afghanistan, Iraq and Libya, that this will end the fighting. Better by far for the G8 leaders to compel all parties in Syria to go to Geneva, agree a ceasefire, establish a UN mission in Syria to monitor it, and then seek to negotiate long-term solutions.