Nearly a century after their first use on a mass scale, the strategic use of chemical weapons in warfare has diminished. The challenge now is to stop the defiance of existing conventions and, in Syria's case, the future control of weapons.
Horrors of chemical warfare in the modern age
On the battlefields of the Somme and Passchendaele, so the story goes, a young gefreiter - or corporal - in the Imperial German Army was so traumatised by his experiences in the Great European War that it changed his views forever.
Serving in the trenches, Adolf Hitler saw first hand the horrors of chemical warfare, and was even briefly blinded by mustard gas in 1918.
As a result, even as the Third Reich collapsed around him, Hitler, now supreme commander - refused all appeals to turn chemical weapons on the advancing Allied forces.
It is a theory with obvious flaws. Hitler's alleged antipathy to chemical weapons is seriously at odds with his use of hydrogen cyanide to speed the extermination of millions of Jews, Roma Gypsies and other "enemies of the race-based state".
More likely is that Hitler and his generals realised that chemical weapons offered no real advantage in modern warfare even in 1939. Their use was more likely to provoke a heavy response from the Allies, with their growing air superiority.
Tactics not ethics is the reason that the gas warfare of the First World War was not repeated in the Second.
Alastair Hay, is a professor of environmental toxicology at the University of Leeds, and an acknowledged expert on the issues relating to both chemical and biological warfare.
As Prof Hay, who has investigated the use of chemical weapons in both Iraq and Bosnia, points out, there were few moral objections over their first use on the battlefield.
"Churchill and others could see little difference between chemical and biological weapons and a bullet," he says.
Hay estimates that out of the nine million military deaths attributed to the 1914-18 war, perhaps 50,000 were the result to chemical warfare - or barely half a per cent of total causalities
Many of those occurred not in the trenches of Western Europe, but against Russian troops in the east, with the British also firing gas shells from tanks at Turkish troops during the Second Battle of Gaza in April 1917.
The legacy of the First World War included thousands of returning soldiers who continued to suffer the lingering effects of exposure to gas warfare for the rest of their sometimes prematurely shortened lives.
Yet while public opinion swung against chemical weapons, they continued to be deployed. During the British rule of Iraq, the use of poison gas was considered against tribes during the 1920 revolt.
At the time, Winston Churchill, then Secretary of State for War, observed: "I do not understand this squeamishness about the use of gas," adding: "I am strongly in favour of using poisoned gas against uncivilised tribes. The moral effect should be so good that the loss of life should be reduced to a minimum."
Churchill was arguing in favour of non-lethal tear gas. Others were less squeamish. Bolshevik forces used poison gas against Russian peasants who rebelled in 1921, while a Berber revolt in 1924 saw Spanish aircraft drop bombs containing lethal mustard gas.
Within a year, the "Geneva Protocol" had been drawn up by the League of Nations, with its provisions banning the use of both chemical and biological weapons coming into force three years later.
Yet the The Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare - to use the full title - did not immediately end chemical warfare or its threat. Many of the great powers, including the US, Russia and Britain, continued to stockpile chemical weapons, while 25 of the signatories insisted on retaining the right to use them in a retaliatory strike.
In the case of Fascist Italy, the pact was openly ignored, with Mussolini sanctioning the use of shells and bombs containing mustard gas against Ethiopia in 1935, resulting in tens of thousands of casualties.
While the moral case against such weapons of mass destruction played an important part in their banning, there is little doubt that many nations were happy to sign up knowing that chemical warfare had not proved its effectiveness on the battlefield.
Prof Hay agrees that the relative ease with which the international community rejected chemical weapons was related to its dwindling strategic value. Whereas the First World War was fought in trenches, the Second involved fast-moving armoured vehicles and aircraft - both targets hard to hit with gas, which requires heavy saturation of a specific location to be effective. Only in the east did Imperial Japan use gas to attack China.
So while nations retained stocks of chemical and biological weapons, there seemed little chance that they would be used. For almost half a century, chemical warfare remained in the shadows.
What incidents took place were too distant to impinge on the world's conscience. There is convincing evidence that Egypt deployed mustard gas in support of royalist forces during the North Yemen Civil War between 1963 and 1966, causing perhaps as many as 1,500 deaths, many of them civilians.
Egypt denied the accusations, saying some of deaths were caused by - legal - napalm, also being widely used by the United States in Vietnam at the time. The United Nations declined an Egyptian invitation to investigate further.
What changed our perception of chemical weapons as a real and current threat, says Prof Hay came in the 1980s with the Iran-Iraq war "and because in the later stages they were used against civilians."
With Iraq losing the war with Iran, Saddam Hussein began more aggressively to develop a chemical and biological weapons programme.
Mustard gas - which causes severe burns and damage to the respiratory system -was first used against the waves of Iranian infantry in 1980, with Iraq developing more sophisticated nerve agents like Sarin by the mid-80s.
Estimates for the number of Iranian casualties run to the tens of thousands, with evidence in recently declassified CIA documents suggesting that the US, at the time fearing Iranian fundamentalism more than Saddam's dictatorship, turned a blind eye to their use and even provided Iraq with intelligence about Iranian attacks.
Then, close to the end of the war, Saddam used his chemical weapons against Kurds in the town of Halabja in March 1988, killing up to 5,000 people and injuring up to 10,000, almost all civilians.
Photographs showed streets filled with bodies, including many children. Far more than distant memories of blinded Tommies on the Somme, these are the images that define the horrors of chemical warfare in the modern age.
Chemical terrorism rather than chemical warfare is a better description of the threat from such weapons nearly a century after their first use on a mass scale. Only five states have failed to ratify the 1997 Chemical Weapons Convention, which bans not just the use of such weapons, but their production and stockpiling. They include Angola, Egypt, North Korea, South Sudan and Syria. Israel and Myanmar (Burma) have signed the convention but not yet ratified it.
Prof Hay says that while "the moral issue has moved on," the challenge now is to stop obvious defiance of existing conventions. As a scientist, he would like to see colleagues in countries like Syria making known their objections to chemical weapons, although he accepts this is difficult.
In the case of Syria, "all the evidence points unquestioningly to the Syrian government using chemical weapons," he believes. But at the same time, for the inspection process to remain credible in the future: "I hope that the UN team is given enough time to do its job."
What happens in the coming days is crucial to the future control of chemical weapons, he says. He is wary of armed force. Bombing suspected stockpiles risks releasing dangerous toxins, and "I dread a military response because it means people getting injured or killed."
Yet at the same time, the international community must show "We are prepared to respond. This about people looking to the future, sending a signal."
"Doing nothing? It comes back to that old Chinese phrase about being a paper tiger."