Chemical warfare
and the Soviet Union entered into a bilateral agreement in June 1990. In the 1990 agreement, the U.S. and Soviet Union agreed to begin destroying their chemical weapons stockpiles before 1993 and to reduce them to no more than 5,000 agent tons each by the end of 2002. The agreement also provided for exchanges of data and inspections of sites to verify destruction. Following the collapse of the Soviet Union, the U.S.'s Nunn–Lugar Cooperative Threat Reduction program helped eliminate some of the chemical, biological and nuclear stockpiles of the former Soviet Union.
The United Nations Conference on Disarmament in Geneva in 1980 led to the development of the Chemical Weapons Convention (CWC), a multilateral treaty that prohibited the development, production, stockpiling, and use of chemical weapons, and required the elimination of existing stockpiles. The treaty expressly prohibited state parties from making reservations (unilateral caveats). During the Reagan administration and the George H. W. Bush administration, the U.S. participated in the negotiations toward the CWC.  The CWC was concluded on September 3, 1992, and opened for signature on January 13, 1993. The U.S. became one of 87 original state parties to the CWC.  President Bill Clinton submitted it to the U.S. Senate for ratification on November 23, 1993. Ratification was blocked in the Senate for years, largely as a result of opposition from Senator Jesse Helms, the chairman of the Senate Foreign Relations Committee. On April 24, 1997, the Senate gave its consent to ratification of the CWC by a 74–26 vote (satisfying the required two-thirds majority). The U.S. deposited its instrument of ratification at the United Nations on April 25, 1997, a few days before the CWC entered into force. The U.S. ratification allowed the U.S. to participate in the Organisation for the Prohibition of Chemical Weapons, the organization based in The Hague that oversees implementation of the CWC.
Upon U.S. ratification of the CWC, the U.S. declared a total of 29,918 tons of chemical weapons, and committed to destroying all of the U.S.'s chemical weapons and bulk agent. The U.S. was one of eight states to declare a stockpile of chemical weapons and to commit to their safe elimination. The U.S. committed in the CWC to destroy its entire chemical arsenal within 10 years of the entry into force (i.e., by April 29, 2007),  However, at a 2012 conference, the parties to the CWC parties agreed to extend the U.S. deadline to 2023. By 2012, stockpiles had been eliminated at seven of the U.S.'s nine chemical weapons depots and 89.75% of the 1997 stockpile was destroyed. The depots were the Aberdeen Chemical Agent Disposal Facility, Anniston Chemical Disposal Facility, Johnston Atoll, Newport Chemical Agent Disposal Facility, Pine Bluff Chemical Disposal Facility, Tooele Chemical Disposal Facility, Umatilla Chemical Disposal Facility, and Deseret Chemical Depot. The U.S. closed each site after the completion of stockpile destruction. In 2019, the U.S. began to eliminate its chemical-weapon stockpile at the last of the nine U.S. chemical weapons storage facilities: the Blue Grass Army Depot in Kentucky. By May 2021, the U.S. destroyed all of its Category 2 and Category 3 chemical weapons and 96.52% of its Category 1 chemical weapons. The U.S. is scheduled to complete the elimination of all its chemical weapons by the September 2023 deadline. In July 2023 OPCW confirmed the last chemical munition of the U.S., and that the last chemical weapon from the stockpiles declared by all States Parties to the Chemical Weapons Convention was verified as destroyed.
The U.S. has maintained a "calculated ambiguity" policy that warns potential adversaries that a chemical or biological attack against the U.S. or its allies will prompt a "overwhelming and devastating" response. The policy deliberately leaves open the question of whether the U.S. would respond to a chemical attempt with nuclear retaliation. Commentators have noted that this policy gives policymakers more flexibility, at the possible cost of decreased strategic unpreparedness.

Anti-agriculture: 

Herbicidal warfare: Although herbicidal warfare use chemical substances, its main purpose is to disrupt agricultural food production and/or to destroy plants which provide cover or concealment to the enemy.
The use of herbicides by the U.S. military during the Vietnam War has left tangible, long-term impacts upon the Vietnamese people and U.S. veterans of the war. The government of Vietnam says that around 24% of the forests of Southern Vietnam were defoliated and up to four million people in Vietnam were exposed to Agent Orange. They state that as many as three million people have developed illness because of Agent Orange while the Red Cross of Vietnam estimates that up to one million people were disabled or have health problems associated with Agent Orange. The United States government has described these figures as unreliable.
During the war, the U.S. fought the North Vietnamese and their allies in Laos and Cambodia, dropping large quantities of Agent Orange in each of those countries. According on one estimate, the U.S. dropped 475,500 US gallons (1,800,000 L) of Agent Orange in Laos and 40,900 US gallons (155,000 L) in Cambodia. Because Laos and Cambodia were officially neutral during the Vietnam War, the U.S. attempted to keep secret its military involvement in these countries. The U.S. has stated that Agent Orange was not widely used and therefore hasn't offered assistance to affected Cambodians or Laotians, and limits benefits American veterans and CIA personnel who were stationed there.

Anti-livestock: During the Mau Mau Uprising in 1952, the poisonous latex of the African milk bush was used to kill cattle.

See also: 1990 Chemical Weapons Accord
Ali Hassan al-Majid
Area denial weapon
Chemical weapon designation
Chemical weapons and the United Kingdom
Gas chamber
List of CBRN warfare forces
List of chemical warfare agents
List of highly toxic gases
Ronald Maddison
Psychochemical weapon
Saint Julien Memorial
Sardasht, West Azerbaijan, a town attacked with chemical weapons during the Iran–Iraq War
Stink bomb
United States Army Medical Research Institute of Chemical Defense

Notes: 

References: CBWInfo.com (2001). A Brief History of Chemical and Biological Weapons: Ancient Times to the 19th Century. Retrieved November 24, 2004.
Chomsky, Noam (March 4, 2001). Prospects for Peace in the Middle East, page 2. Lecture.
Cordette, Jessica, MPH(c) (2003). Chemical Weapons of Mass Destruction. Retrieved November 29, 2004.
Croddy, Eric (2001), Chemical and Biological Warfare, Copernicus, ISBN 978-0-387-95076-1
Smart, Jeffery K., M.A. (1997). History of Biological and Chemical Warfare. Retrieved November 24, 2004.
United States Senate, 103d Congress, 2d Session. (May 25, 1994). The Riegle Report. Retrieved November 6, 2004.
Gerard J Fitzgerald. American Journal of Public Health. Washington: Apr 2008. Vol. 98, Iss. 4; p. 611
Гречко, А.А. (1976). Годы Войны. Военное Издательство Министерства Оборонны СССР.Москва.

Further reading: Leo P. Brophy and George J. B. Fisher; The Chemical Warfare Service: Organizing for War Office of the Chief of Military History, 1959; L. P. Brophy, W. D. Miles and C. C. Cochrane, The Chemical Warfare Service: From Laboratory to Field (1959); and B. E. Kleber and D. Birdsell, The Chemical Warfare Service in Combat (1966). official US history;
Glenn Cross, Dirty War: Rhodesia and Chemical Biological Warfare, 1975–1980, Helion & Company, 2017
Gordon M. Burck and Charles C. Flowerree; International Handbook on Chemical Weapons Proliferation 1991
L. F. Haber. The Poisonous Cloud: Chemical Warfare in the First World War Oxford University Press: 1986
James W. Hammond Jr; Poison Gas: The Myths Versus Reality Greenwood Press, 1999
Jiri Janata, Role of Analytical Chemistry in Defense Strategies Against Chemical and Biological Attack, Annual Review of Analytical Chemistry, 2009
Ishmael Jones, The Human Factor: Inside the CIA's Dysfunctional Intelligence Culture, Encounter Books, New York 2008, revised 2010, ISBN 978-1-59403-382-7. WMD espionage.