How can we know that a chemical weapons attack took place in Syria?

Every atrocity in the Syrian civil war provokes a furious row about whether it happened and, if so, who was responsible for carrying it out. The merciless brutality of all sides combines with partisan reporting and lack of access for independent investigators to make it possible for doubts to be generated about even the most blatant war crime. One good rule is that participants in the war are often accurate about the crimes of their opponents while they invariably lie or are silent about their own.

This rule appears to hold good in the case of the poison gas attack on the city of Douma on 7 April, which killed at least 34 people and possibly twice as many. The Russian military claim that the attack was faked by pro-opposition activists and that samples taken from the site of where the civilians died were not toxic. The Syrian government issues blanket denials when accused of using poison gas.

But there is mounting evidence from neutral observers to confirm that chlorine was used last Saturday. The World Health Organisation says that local health authorities in Douma, with whom it is cooperating, confirm that on the day of the alleged bombing they treated 500 patients with the symptoms of exposure to toxic chemicals. It reports that "there were signs of severe irritation of mucous membranes, respiratory failure and disruption to the central nervous systems of those exposed".

Other evidence for the gassing of civilians is cumulatively convincing: large gas cylinders, like those used in past chlorine gas attacks, were filmed on the roof of the building where most bodies were found. Local people report that Syrian government helicopters were seen in the area at the time of the attack. Such helicopters have been used in chlorine gas bombings in the past.

The Russian and Syrian government accounts of what happened, varying between saying there were no attacks or that evidence for them has been fabricated, are contradictory. A Russian spokeswoman said on Wednesday that the use of "smart missiles" on Syrian government forces could be an attempt to destroy the evidence.

The allegations of fabrication are generalised and non-specific and amount to a conspiracy theory for which no evidence is ever produced, other than to throw doubt on the partiality of those who say that chlorine was used. It is true that many of the sources cited by the Western media as if they were bipartisan eye-witness accounts are committed supporters of the opposition. But the Russian and Syrian governments have never produced any counter-evidence to give credence to the elaborate plot that would be necessary to fake the use of poison gas or to really use it, but put the blame on Syrian government air power.

The most convincing reason advanced by those who argue that President Bashar al-Assad’s forces did not carry out the attack is that it was entirely against their interests to do so. They have already won militarily in Douma and the second of two convoys carrying thousands of Army of Islam fighters and their families left for Turkish-controlled northern Syria today. And this latest success brings Assad with sight – though it is still a distant one – of a complete victory over his enemies.

For all the furore about the proposed missile strike on Syrian forces – likely to happen in the very near future – it is difficult to see what it will achieve other than as a general sign of international disapproval of the use of chemical weapons. Hawks in the US and Europe may want to use the occasion to reopen the door to armed intervention in the Syrian civil war with the aim of weakening or displacing Assad, but the time for this is long past, if it was ever there.

There is a widely held myth that US air strikes against government forces in 2013, which President Barack Obama is blamed for not having carried out, would have brought the war to a different and happier conclusion. But such air strikes would only have been effective if they had been conducted on a mass scale and on a daily basis in support of ground troops. These would either have been Sunni Arab armed opposition forces, which were already dominated by al-Qaeda-type movements, or the US army in a rerun of the Iraq War of 2003.