Short answers, pulled from the story.
Chemical warfare involves using the toxic properties of chemical substances as weapons regardless of their origin. Under the Chemical Weapons Convention, any toxic chemical is considered a chemical weapon unless it is used for purposes that are not prohibited.
Fritz Haber is considered the father of chemical warfare for his years of pioneering work developing and weaponizing chlorine and other poisonous gases during World War I. Germany became the first side to employ chemical warfare on the battlefield by opening canisters of chlorine upwind of the opposing side.
Historians estimate that between 500,000 and 1.3 million casualties were directly caused by chemical warfare agents during the course of the war. Of these casualties, 2-4% died while at least 1,300 civilians were injured due to the use of the weapons.
The Senate ratified the Chemical Weapons Convention in April 1997 after Dick Cheney opposed its signing ratification in a letter dated the 8th of April 1997. By September 2017, Russia had destroyed its entire chemical weapons stockpile and over 76 percent or 30,500 tons of declared stockpiles were destroyed as of August 2013.
The Ba'athist regime in Syria used sarin, chlorine, and mustard gas in numerous deadly chemical attacks against civilian populations during the Syrian civil war. During the Russian invasion of Ukraine, Russia deployed CS gas through K-51 grenades dropped by unmanned drones resulting in over 4,800 incidents recorded since the war began.