First World War
Chemical weapons were deployed for the first time in warfare. Chlorine and phosgene gases were released from canisters and dispersed by the wind. It is estimated that 90,000 died as a result. In 1916 the British government established the Porton Down facility in Wiltshire to research and develop chemical weapons.
Britain's invasion of Soviet Russia
In 1919 Winston Churchill authorised the use of chemical weapons in northern Russia as part of the attempt to overthrow the new Soviet regime.
The Italian army used mustard gas during its invasion of the African country.
A US ship that was hit by a bomb off the coast of Italy leaked mustard gas, killing 1,000.
Enormous stockpiles of chemical weapons were held by both the US and the Soviet Union during the Cold War - enough to destroy most of human and animal life on earth.
A British soldier, Ronald Maddison, died after a sarin gas experiment at Porton Down. A campaign by his family finally succeeded in having his death ruled unlawful in 2004.
US forces dropped 20 million gallons of Agent Orange, a toxic defoliant, on Vietnam between 1962 and 1971. At least 150,000 Vietnamese children have been born with birth defects as a result of the use of Agent Orange.
Iran-Iraq war 1980-1988
Declassified CIA documents show that the US knew of Iraqi use of chemical weapons as early as 1983. The US backed and armed Saddam Hussein's regime throughout this period. When the regime used chemical weapons against the Kurdish city of Halabja, leaving up to 5,000 dead, the US took no action.
US war on Iraq
US forces used white phosphorus mixed with high explosives during its siege of the Iraqi city of Falluja in November 2004. The US and Britain also used depleted uranium shells in Iraq, including in Falluja. A study of households in Falluja between 2005-2009 found abnormally high rates of cancer and birth defects. One the study's authors said they found "the highest rate of genetic damage in any population ever studied."
Israeli attack on Gaza, 2009
Israeli used white phosphorus bombs, exploded from the air, when it attacked the densely populated Gaza strip in 2009.