Phosgene gas was used extensively by both sides during the war. It was typically released from cylinders or artillery shells, and it could spread quickly over a wide area. Phosgene gas was particularly effective in trench warfare, where it could be used to incapacitate or kill enemy soldiers without having to risk direct combat.
The use of phosgene gas was controversial, as it was seen as a particularly inhumane weapon. The gas caused great suffering to those who were exposed to it, and it often led to death. In an effort to reduce the use of chemical weapons, the Geneva Protocol was signed in 1925, which banned the use of chemical weapons in warfare.