Poison gas was first invented and used in warfare by the French military in 1793, during the Battle of Fleurus. The gas, which was chlorine, was released from cylinders and caused severe respiratory problems and death. However, it was not until World War I that poison gas was used on a large scale, by both the Allies and the Central Powers.

There are a number of reasons why poison gas was invented and used in warfare. First, it was a relatively inexpensive way to kill or injure large numbers of people. Second, it was a way to overcome the stalemate that had developed on the Western Front, where both sides were entrenched in deep trenches and unable to advance. Third, it was a way to terrorize the enemy and break their morale.

Poison gas was used in a variety of ways during World War I, including:

* Shells: Gas shells were filled with a variety of chemicals, including chlorine, phosgene, and mustard gas. These shells were fired at enemy trenches and fortifications, releasing the gas and causing casualties.

* Gas cylinders: Gas cylinders were placed on the front lines and opened, releasing the gas. This method was used to create a gas cloud that would drift over enemy positions.

* Gas projectors: Gas projectors were devices that were used to project gas clouds over enemy positions. These projectors could be mounted on vehicles or airplanes.

Poison gas caused widespread casualties during World War I. An estimated 1.3 million people were killed by gas, and many more were injured. The use of gas led to the development of gas masks and other protective measures, but it did not prevent the gas from being used again in World War II and other conflicts.

After World War I, the use of poison gas was banned by the Geneva Protocol, which was signed in 1925. However, poison gas has continued to be used in some conflicts, such as the Iran-Iraq War and the Syrian Civil War.