The military use of herbicides in Vietnam began in 1961, was expanded during 1965 and 1966, and reached a peak from 1967 to 1969. Herbicides were used extensively in Vietnam by the U.S. Air Force's Operation RANCH HAND to defoliate inland hardwood forests, coastal mangrove forests, and cultivated land, by aerial spraying from C-123 cargo/transport aircraft and helicopters. Soldiers also sprayed herbicides on the ground to defoliate the perimeters of base camps and fire bases; this spraying was executed from the rear of trucks and from spray units mounted on the backs of soldiers on foot. Navy riverboats also sprayed herbicides along riverbanks. The purpose of spraying herbicides was to improve the ability to detect enemy base camps and enemy forces along lines of communication and infiltration routes. Spraying was also used to destroy the crops of the Viet Cong and North Vietnamese.
The code name for the overall herbicide program was TRAIL DUST. The code name RANCH HAND specifically referred to the C-123 herbicide-spraying project.
The different types of herbicide used by U.S. forces in Vietnam were identified by a code name referring to the color of the 4-inch band painted around the 55-gallon drum that contained the chemical. These included Agents Orange, White, Purple, Blue, Pink, and Green. e.g. A 55-gallon drum with an orange band contained 50% n-butyl ester of 2,4-D (2,4-dichlorophenoxyacetic acid) and 50% n-butyl or isooctyl ester of 2,4,5-T (2,4,5-trichlorophenoxyacetic acid).
Agent Orange accounted for over 60% of the total herbicides disseminated over Vietnam (11.7 million gallons of a total 19.4 million gallons).
Orange contained relatively high levels of an exceedingly poisonous contaminant known as "dioxin" or "TCDD" (2,3,7,8-tetrachlorodibenzo-p-dioxin)