A 'one hundred year taboo'? One graphic that shows how chemical weapons have been used over time
The use of one of the world's deadliest weapons - Novichok - to poison Sergei Skripal and his daughter, Yulia, on British soil has sparked global alarm over chemical weapons.
Such weapons have been used in warfare throughout history - but the first large-scale use of chemical weapons took place just over one hundred years ago during World War One, where both sides used gases including chlorine, phosgene and mustard gas against their enemies on the battlefield.
After this, the twentieth century saw several instances of use of chemical and biological weapons by both states and terrorist groups.
Below, The Telegraph charts some of the major instances of use of chemical and biological weapons in the last 100 years, along with the major efforts to try and restrict their use.
Use of chemical and biological weapons since 1915
Following World War One, the 1925 Geneva Protocol reaffirmed the Hague Convention’s earlier ban on use of asphyxiating, poisonous or other gases and bacteriological methods of warfare. The Geneva agreement stopped short however, of banning experimentation, production and storage of chemical and biological weapons.
Although conventions against the use of chemical and biological agents have failed to stop their use completely, widespread fear towards chemical and biological weapons meant that - with the exception of Japan - parties to World War Two refrained from using these during the conflict.
More recently however, President Assad has repeatedly used such weapons against his own people during the Syrian conflict, representing a clear contravention of the 1993 Chemical Weapons Convention - which Syria joined in 2013 .
Despite the 2013 deal brokered by Russia and the USA that required Syria to destroy its chemical weapons stockpiles, the UN has verified some 30 instances of the use of chemical weapons during the Syrian conflict so far, mostly by Assad’s forces.
But many reports of number of instances of use in Syria run even higher.
Chemical warfare in Syria threatens to undermine the longstanding norm - referred to by experts as the "one hundred year taboo" - against the use of chemical and biological weapons.
Despite repeated warnings from several countries including the United States and France that use of chemical weapons represents a "red line" in Syria, no action has been taken yet against the Assad regime.
