Gas was perhaps first used in battle in the Peloponnesian War between Sparta and Athens(431-404 BC) when burning pitch and sulfur were used to produce suffocating gases. But poison gas as a modern weapon was introduced in the First World War on April 22, 1915. The Germans then launched a cylinder attack with chlorine against the British and French at Ypres.
Later many varieties of war gas were introduced. In July 1917 the Germans began using mustard gas, which caused severe, slow-healing burns on the skin and damage to the lungs. Many casualties resulted from gas bombardment, but relatively few people died. Both sides were ready to use increased gas attacks when the war ended in 1918.
During the Second World War and the Korean War, gas was not used, but preparations were carried out by both sides. Perhaps the most terrifying discovery was made by the Germans. This was a new series of chemical agents called nerve gases which rapidly result in convulsions, coma and - unless promptly treated - death.
Tear gas, which causes intense smarting and watering of the eyes is used in many countries to control rioting crowds, but its effects are only temporary.