Did Syria gas its own people? The evidence is mounting...

The priority for Syrian foreign policy for the past two-and-a-half years has been to avoid foreign military intervention on behalf of the rebels. By the same token, the opposition has tried by every means to secure armed intervention by the US and its allies sufficient to win the war.

The action by the Syrian government most likely to push an unwilling White House into military involvement has been the open use of chemical weapons against civilians. Damascus has furiously denied in the past that it had done so and proof has been lacking. Rebel accusations might have been fabricated and claims by Western governments were tainted by propaganda.

Experts specialising in chemical weapons had hitherto expressed scepticism, even derision, at supposed proofs of chemical weapons use in the media. CBRNe World, a journal specialising in chemical and biological weapons, asked of one alleged sarin gas attack: "Could it be real – possibly. Could it be misdiagnosed and something other than sarin – possibly. Could it be fake – possibly." Considering the question two months ago of whether chemical weapons had been used in Syria, Professor Julian Perry Robinson of Sussex University, a renowned expert, concluded: "Onlookers can as yet believe the reporting only if they are willing to trust unsubstantiated assertion or incomplete evidence."

So it is difficult to think of any action by the Damascus government more self-destructive than the Syrian army launching a massive chemical-weapons attack on rebel-held districts in its own capital. Yet the evidence is piling up that this is exactly what happened last Wednesday and that the Syrian army fired rockets or shells containing poison gas which killed hundreds of people in the east of the city. The opposition may be capable of manufacturing evidence of government atrocities, but it is highly unlikely it could do so on such a large scale as this.

President Obama's security advisers were meeting yesterday in the White House with the strong possibility that there will be a US military response, such as missile strikes from outside Syrian airspace on Syrian military units or bases from which the chemical weapons may have been launched. No doubt Obama would like to keep out of a full-scale intervention, as he made clear last week, saying of the wars in Iraq and Afghanistan that people who "call for immediate action, jumping into stuff that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region". Nevertheless, the blatancy of the poison-gas attacks will make it difficult and damaging for him not to react militarily.

If the Syrian leadership knew that chemical weapons were going to be used, what could be their motive? They may be so convinced of American weakness and so confident of the backing of Russia and Iran that they feel they can ignore international condemnation. They may have seen Egypt's security forces shoot down hundreds of Muslim Brotherhood supporters on 14 August and thought, "If they can get away with it, so can we." Even so, the benefits of such an operation were always going to be outweighed by political costs abroad.

Other factors, too, may have been at work. The Middle East has been bubbling, this past year, with exaggerated talk of US political and military decline, pumped up by visits from US politicians such as Senator John McCain denouncing White House "cowardice". No doubt the US has a weaker position in the Middle East because of the Iraq and Afghan wars, when its army failed to defeat limited guerrilla forces. But US and Nato intervention in Kosovo in 1999 was cited last week as an example of interventions that succeeded.

Still, the Balkans are different from the wider Middle East where American interventions have usually brought disaster. There was bloody failure in Lebanon in 1982-83 and in Somalia in 1993; and even the one exception, the First Gulf War in 1991, did not turn out so well for the US in the long term. Moreover, the failure of the Iraq war of 2003 and the ongoing Afghan conflict have soured American voters' enthusiasm for other Middle East ventures.

Syria's conflict differs from Iraq, Afghanistan and Libya in another respect: Moscow is back as a world power and cannot be ignored or intimidated. One of the reasons Saddam Hussein invaded Kuwait in 1990 was the calculation that the fall of the Soviet Union would leave the US as sole superpower and cramp independent action by Iraq or other regional powers. Russia is back for the first time in more than 20 years as a powerful player, embittered by what it understandably sees as a double-cross over Nato intervention in Libya and determined not to let that happen again.

Russia's re-emergence is not the only factor restraining America. For all the wringing of hands in Washington and Western Europe about the human tragedy, the present situation is not entirely against their interests. Syria, so long the heart of opposition to the West and Israel in the Arab world, is, for now, fragmented and weak. Any decisive outcome ending the war carries with it clear risks for Western interests. If President Bashar al-Assad wins then this is a defeat for them and a success for Iran and Hezbollah. If he is defeated then al-Qa'ida-linked organisations, such the al-Nusra Front and the Islamic State of Iraq and the Levant – increasingly the most effective military component of the opposition – will be among those who replace him. The US, Britain and France may want the war to end but only on their own terms – probably involving the decapitation of the government, but stopping well short of revolutionary change.

What will be the impact of the chemical-weapons attack within Syria? It will frighten people further in rebel areas and will show the utter ruthlessness of the government, something scarcely in doubt. But the action is also a sign of weakness, suggesting the Syrian army cannot capture with conventional arms districts such as Jobar close to the centre of Damascus. Plenty of Syrian officials can see the criminal stupidity of using chemical weapons, so experts are asking if some state faction might want to sabotage possible peace talks by deploying them. A problem with this scenario is nobody else has noticed peace talks getting anywhere.

The Syrian government denies it had anything to do with the gas attack, but it has not given a credible account of what did happen. Initially, there was disbelief that it would do something so patently against its own interests, but all the evidence so far is that it has done just that.