Our thoughts of chemical warfare automatically gravitate to the horrors of World War I and the mustard gas immortalised by photographs of lines of blind men and the poems of some of the war poets. However there is new evidence from Dura-Europos, the great Roman fortress in modern day Syria. It was besieged by the Persians in the 3rd Century AD,and has recently provided some startling finds which may point to the use of chemical warfare over 1700 years before WWI. Read about it here.