During the Vietnam War, U.S. forces used defoliants to deprive North Vietnamese forces of cover. These substances caused leaves to fall off plants when applied en masse over forests. The most infamous defoliant used during the war was Agent Orange, a mixture of two compounds called 2,4,5-T and 2,4-D in a 1:1 ratio. Exposure to Agent Orange led to major illnesses in over a million people in Vietnam after the war.
Britain was the first country to use defoliants as a weapon of war during the Malayan Emergency. They used a substance similar to Agent Orange, but with the addition of a toxin called Dioxin. Dioxin was responsible for the Seveso disaster of 1976.
The United States also developed a mixture of cacodylic acid, sodium cacodylate, and water to destroy rice plants during the Vietnam War. These plants were difficult to destroy even with fire, but the mixture dehydrated and killed them. The use of defoliants against civilian populations has been banned since 1980, but civilians were affected during the Vietnam War.
Napalm, a substance that burns easily and spreads quickly, was famously photographed in 1972 by Nick Ut during the Vietnam War. At a U.N. conference in June 1972, Olof Palme described the effects of defoliants in Vietnam as “ecocide” and advocated for it to be designated an international crime.