It was April 1915 that chemical weapons were introduced as a instrument of modern warfare.
All sides would come to use the weapons dubbed "the cloud that kills", but it was the Germans that used it first, against Allied troops in what became known as the battle of Ypres, in Belgium.
A thick green gas of chlorine gas released by the Germans poured over bunkers and killed soldiers inside the place they ran to hide.
Paul Davies reports on the devastating impact of that initial offensive.
Chemical weapons expert Hamish de Bretton-Gordon said the psychological impact the use of the gas would have had on commanders and soldiers is difficult to underestimate.
Allied forces were taken completely by surprise, and left with nowhere to hide.
Since then, the potency of chemical weaponry has increased exponentially, but the psychological image remains largely the same.
In Syria today it is civilian populations who are experiencing the horror of being poisoned from the air, despite their usage being made illegal under the international rules of war.
Chemical weapons, Mr de Bretton-Gordon have become a "battle-winning capability" for President Assad, as they are so effective for fighting in cities.
The physical impacts of the weapons are also exactly the same now as they were 100 years ago, Mr de Bretton-Gordon, who has recently returned from Syria, said.
He said the children he met in Syria, suffering from the effects of asphyxiation and the burning of their lungs, died from the same injuries sustained by soldiers 100 years ago on the battle fields of the First World War.