The Syrian conflict has raged for more than two years, claiming more than 100,000 lives and drawing international condemnation.
But it took an alleged chemical weapons attack on a Damascus suburb which the opposition says killed hundreds of people to push Western powers towards military strikes.
“This case shows the very special place that chemical weapons occupy in the panoply of arms,” said Olivier Lepick, a researcher at the Paris-based Foundation for Strategic Research.
The First World War was the first modern chemical war, and remains the “mother of all chemical wars”, said Lepick.
In April 1915, 15,000 Allied soldiers were killed when the German army used tons of chlorine gas near Ypres in Belgium.
Allied governments called the attack a flagrant violation of international law, but Germany argued that the law only banned chemical shells, rather than the use of gas projectors.
“The unprecedented use of gas by the Germans was immediately seen as a war crime — all the more so because it does not lead to bleeding or mutilated bodies like shells and bullets,” said historian Annette Becker.
“Gas is considered one of the greatest killers during World War I, although it claimed tens of thousands of lives against hundreds of thousands caused by shells and bullets.”
The allies’ revulsion, however, did not stop them from resorting to such weapons, and the notorious “mustard gas” attacks left a strong mark on the collective imagination.
The horrors of that conflict led to strong calls for chemical weapons to be outlawed, and in 1925, the Geneva Convention banned the use of chemical and biological weapons — but not their production.
It was not until after the 1980s, when former Iraqi leader Saddam Hussein began deploying chemical weapons against Iran before launching a major attack on the Iraqi Kurdish village of Halabja, killing thousands, that a total ban was introduced, and the 1993 Paris Convention outlawed their production, storage and transfer.
In another prominent case, Agent Orange was deployed by the US military in Vietnam to destroy food crops and the vegetation the enemy used as cover, after it was authorised by the administration of president John F. Kennedy in 1961.
Although intended as a herbicide, it had a catastrophic effect on the health of soldiers and civilians.
The ban on chemical weapons dates back to Roman law, which barred the use of poison on weapons. In the 19th century, toxic arms and projectiles loaded with gas were banned by the Brussels Convention and The Hague Convention.
Today, it is not so much the contravention of international law as the moral imperative and the threat of a domino effect that the United States is eager to stress.
US President Barack Obama said long before last week’s alleged attack that the use of chemical weapons in the conflict was a “red line”, while his Secretary of State John Kerry has called the suspected use of the banned weapons a “moral obscenity”.
Experts say it is the indiscriminate nature of chemical weapons, and the suffering they cause, that makes them so reviled.
“A shell in theory targets a particular position, whereas when one uses hundreds of litres of gas, it’s not a surgical strike,” said Lepick.
Emmanuel Goffi, an air force officer who teaches international law, said that if there have to be victims in war, “the aim is to see that people die without without needless suffering”.
“When people suffer for a long time it