Short answers, pulled from the story.
Chemical warfare is the use of the toxic properties of chemical substances as weapons, distinct from nuclear, biological and radiological warfare, which together make up the CBRN category of weapons of mass destruction. About 70 different chemicals have been used or stockpiled as chemical warfare agents, and the 1993 Chemical Weapons Convention bans them outside narrow research, medical or protective purposes.
Modern chemical warfare began during World War I, when Germany became the first side to use it on the battlefield by releasing chlorine gas from open canisters upwind of enemy trenches. The French responded with phosgene-filled artillery shells, and historians estimate 500,000-1.3 million battlefield casualties from chemical agents by the war's end.
Nazi Germany researched and stockpiled nerve agents throughout World War II but never used them, even in its final defense. Adolf Hitler's own experience with poison gas in World War I, fear of Allied retaliation, and the lack of any known antidote, according to military historian Ian V. Hogg, are the cited reasons.
In the 1988 Halabja chemical attack, Iraq used mustard gas and nerve agents against the Kurdish population, killing more than 5,000 people and injuring many more. It came during the wider Iran-Iraq War, in which roughly 100,000 Iranian troops were casualties of Iraqi chemical weapons.
Russia has reportedly deployed CS gas through K-51 grenades dropped by unmanned drones during its invasion of Ukraine. Ukraine's military said on the 13th of December 2024 that over 4,800 chemical weapons incidents had been recorded since the war began, hospitalizing more than 2,000 soldiers and killing three.
India was the first of six declared possessor states to meet its destruction deadline, telling the United Nations on the 14th of May 2009 that it had completely destroyed its chemical weapons stockpile. Russia and the United States, which held far larger arsenals, did not finish until September 2017 and July 2023 respectively.