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Herbicidal warfare is a form of chemical warfare in which the objective is to destroy the plant-based ecosystem of an area. Its purpose is to disrupt agricultural food production and/or to destroy plants which provide cover or concealment to an enemy. Herbicidal warfare has been forbidden by the Environmental Modification Convention since 1978, which bans "any technique for changing the composition or structure of the Earth's biota".
Modern day herbicidal warfare resulted from military research discoveries of plant growth regulators in the Second World War, and is therefore a technological advance on the scorched earth practices by armies throughout history to deprive an enemy of food and cover.
Work on military herbicides began in England in 1940, and by 1944 the United States joined in the effort. Even though herbicides are chemicals, due to their mechanism of action (growth regulators) they are often considered a means of biological warfare. Over 1,000 substances were investigated by wars end for phytotoxic properties, and the Allies envisioned using herbicides to destroy Axis crops. British planners did not believe herbicides were logistically feasible against Germany, and United States' plans for reducing Japanese rice crops for the invasion of Japan were rejected due to the similarity of the herbicide's name to the poison cyanide.
The British first used herbicides against Malaya's insurgency in 1953 by using 2,4,5-T that was intended to control rubber tree parasites. It was used to thin jungle trails to limit ambushes, and destruction of native agriculture. The denial of food was considered a decisive weapon in countering the insurgency, but was later judged to be ineffectual and contrary to other goals.
Vietnam War 
- See also: Operation Ranch Hand
The first major use of herbicides against people in a conflict was by the United States in Southeast Asia during the Vietnam War. Success with Project AGILE field tests with herbicides in South Vietnam in 1961 led to the formal herbicidal program Trail Dust (1961–1971). Operation Ranch Hand, an Air Force program to use C-123K aircraft to spray herbicides over large areas was one of many programs under Operation Trail Dust. The aircrews charged with spraying the defoliant used a sardonic motto-"Only you can prevent forests"-a shortening of the U.S. Forest Services famous warning to the general public "Only you can prevent forest fires".
Initial operations from 1961-1962 were disguised as being under South Vietnamese command, to avoid conflicts with the Geneva Accord of 1954, which prohibited chemical weapons. (The United States officially claims that herbicidal weapons and incendiary agents like napalm fall outside the Geneva definition of "chemical weapons"). Ranch Hand started as a limited program of defoliation of border areas, security perimeters, and crop destruction. As the conflict continued, the anti-crop mission took on more prominence, and (along with other agents) defoliants became used to compel civilians to leave Viet Cong-controlled territories for government-controlled areas. It was also used experimentally for large area forest burning operations that failed to produce the desired results.
Defoliation was judged in 1963 as improving visibility in jungles by 30 - 75% horizontally, and 40 - 80% vertically. Improvements in delivery systems by 1968 increased this to 50 - 70% horizontally, and 60 - 90% vertically. Ranch Hand pilots were the first to make an accurate 1:125,000 scale map of the Ho Chi Minh trail south of Tchpone, Laos by defoliating swaths perpendicular to the trail every half mile or so.
Types of herbicides 
The United States had technical military symbols for herbicides that have since been replaced by the more common color code names derived from the banding on shipping drums.
In 1966 the United States Defense Department claimed that herbicides used in Vietnam were not harmful to people or the environment. In 1972 it was advised that a known impurity precluded the use of these herbicides in Vietnam and all remaining stocks should be returned home. In 1977 the United States Air Force destroyed its stocks of Agent Orange 200 miles west of Johnston Island on the incinerator ship M/T Vulcanus. The impurity, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was a suspected carcinogen that may have affected the health of over 17,000 United States servicemen, 4,000 Australians, 1,700 New Zealanders, Koreans, countless Vietnamese soldiers and civilians, and with over 40,000 children of veterans possibly suffering birth defects from herbicidal warfare.
Decades later the lingering problem of herbicidal warfare remains as a dominant issue of United States-Vietnam relations. In 2003, a coalition of Vietnamese survivors and long-term victims of Agent Orange sued a number of American-based and multinational chemical corporations for damages related to the manufacture and use of the chemical. A federal judge rejected the suit, claiming that the plaintiff's claim of direct responsibility was invalid.
See also 
- E14 munition
- E77 balloon bomb
- M115 bomb
- U.S. Army Biological Warfare Laboratories
- War on Drugs#Aerial herbicide application
- "Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques". UN. 10 December 1976. Retrieved 9 June 2012.
- "How Britain sprayed Malaya with dioxin", New Scientist 19 January 1984, p. 6 full text
Further reading 
- Martini, Edwin A. (2013). "Hearts, Minds, and Herbicides: The Politics of the Chemical War in Vietnam". Diplomatic History 37 (1): 58–84. doi:10.1093/dh/dhs003.
- Westing, A. H. (1989). "Herbicides in Warfare: The Case of Indochina". In Bourdeau, P.; et al. Ecotoxicology and Climate. Stanford University, Department of Global Ecology.