Poison gases were first used in modern warfare during the First World War. On April 22, 1915, the first large scale use of gas as a weapon was used by the Germans in the Langemarck sector of the Ypres salient in an attempt to break the allied line. Algerian troops (part of the French Army) were the first to be overcome by choking clouds of chlorine gas. Although the Algerians broke and ran, the breakthrough was only temporary as the gap was quickly plugged by Canadians fighting with the British. From this first instance began the use of gas as an offensive weapon.
Other gases employed later in the war were mustard and phosgene. Both were capable of killing; mustard by burning the lungs, causing death by drowning in one's own fluids; phosgene, like chlorine, by asphyxiation.
Contrary to popular belief, gas warfare was banned because it was found to be ineffective, not because its effects were so hideous. Effective defensive measures were quickly developed for each new gas, substantially reducing casualties for following attacks. In addition, gas kills attacker as well as defender - it shows no loyalty.
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