SYRIA

The evidence that sarin nerve gas was used against civilians in the rebel-held town of Khan Sheikhoun largely comes from reports (from Turkish doctors) who treated survivors of the Syria attack earlier this month.

Victims were choking, foaming at the mouth, defecating and vomiting – all of which are consistent with sarin use.

Sarin, a colourless, odourless liquid at room temperature, is expensive and complex to manufacture.

The two key chemical compounds – a phosphorus variant and isopropyl alcohol – are mixed near the point of use, usually hours before it is released.

This is to avoid accidents and degradation in storage. The level of sophistication required in handling sarin would suggest state involvement.

Syrian president Bashar al-Assad was supposed to have surrendered his entire chemical weapons stockpile – including sarin – to Russia after an earlier attack on an opposition-held area near Damascus in 2013. More than 1,000 victims died and only a Russian-brokered deal – with Assad agreeing to give up his chemical weapons for destruction – prevented US airstrikes then. According to some reports which have now surfaced, a consignment of sarin was missing from the stockpile handed over.

At the same time, Assad signed up to the Chemical Weapons Convention, a group of states which ban these weapons. However, chlorine gas, which produces similar symptoms to sarin, was not covered by the removal deal. And unlike sarin (which is 3,000 times more lethal) chlorine is easily accessible and has many everyday uses.

Medecins Sans Frontieres doctors, who treated some victims, have said that both a toxic nerve agent and chlorine may have been used. But until impartial experts establish whether, and what, chemical weapons were involved, sole reliance on the observations of doctors is insufficient.

Central to the issue for many is why Assad would use chemical weapons in a war that he’s clearly winning? It is a perplexing question. Since September 2015, when the Russians first intervened in Syria, Assad’s regime has made steady progress in defeating various rebel opponents, notably when his forces took Aleppo in December.

• See also: The powderkeg of Syria

In recent days, the US has strongly suggested it was prepared to leave Assad in power, as it saw him as a potential ally in the fight against Islamic State. Syria’s military continue to categorically deny that it was responsible for the attack, but, of course, Assad has used various weapons indiscriminately against civilians, including barrel bombs (dropped from helicopters) and unfocused artillery bombardment. He has also ‘weaponised’ gases – for example, putting tear gas in shells used by police to quell rioters.

Many are likely to believe, however, that Assad would have to be insanely overconfident to have brazenly used sarin, not least because of the risk – since realised – of heavier US reprisals and greater involvement in the area. All the evidence is that this cruel and calculating man is not insane.

He has remained intent, though, on corralling the remaining rebels in Idlib province where the attack took place. This act of terror may have been a signal that he felt he could act with impunity, particularly following the call by the US Ambassador to the UN that America was no longer seeking for the Syrian president to stand down.

The natural follow-on question is if not Assad, then who was it and why?

Charges of using chemical weapons are a very useful propaganda tool to blacken the reputation of any opponent, however dark already. Conspiracy theorists will see various nefarious hands at work.

The Russians, who back Assad’s regime, claim the Syrian air force bombed chemical munitions held by rebel forces in a warehouse, which then exploded. Another claim is that it was a gas manufacturing plant.

Such a strike would probably have destroyed what sarin there was and distributed the rest over a smaller area, affecting fewer victims.

Given that the highly flammable isopropyl alcohol is one of the chemicals in sarin, a fireball might have been expected but there have been no reports of this.

The numbers of women and children caught up in the attack would also rule against a rebel-held munitions depot in the immediate area.

Sarin can be delivered via shells, but some witnesses saw ‘chemical bombs’ falling. The first reports from the site described a crater where a chemical-bearing rocket was said to have landed. There were no structural remains suggesting an explosion at a warehouse.

While it is possible that rebel forces acquired the chemicals to make sarin, or other nerve agents, these are unlikely to have been in large enough quantities to cause so many casualties.