1914-Fritz Haber developed poison gases for Germany during the First World War including: chlorine gas and phosgene, among others.
1915-Chlorine gas was first released at Ypres, Belgium by the Germans
1916- Gases became a standard weapon and were used enthusiastically throughout the war
1917- The Germans fired the chemical shells in the Battle of Ypres which let off a brown, oily fluid and a rancid mustard smell (Mustard Gas).
1918- W. Lee Lewis, an american scientist, developed the chemical Lewisite.
1920's and 30's- Many chemicals were developed including; Tabun, Sarin, the Nitrogen Mustards, and Phosgene Oxime
1925- The Geneva Protocol was created, banning the use of chemicals in war but not banning the stockpiling of the weapons themselves.
1939 to 1945- During World War II gases were not used in the battles but by Adolf Hitler to kill the Jews he persecuted.
1940's and 50's- After WWII more chemicals were developed including; Soman, VX among others.
1962-1970- U.S. military sprayed agent orange on about 10 percent of the area of southern Vietnam, during the Vietnam War.
1974- The U.S ratified the Geneva Protocol
1980-1988- In the war between Iraq and Iran, Iraq used mustard and nerve agents.
1993- a international treaty banning the production, stockpiling (both by 2007), and use of chemical weapons and calling for the establishment of an independent organization to verify compliance was adopted.
1997- The treaty mentioned previously came into effect