United States military . The land war in Vietnam raged across much of South Vietnam , and the United States tried to make a battlefield more favorable to them . According to a number of estimates , the Air Force sprayed herbicides on one tenth of the land mass of South Vietnam , about 6600 square miles (Young , A . L , J . A . Calcagni , C . E . Thalken , et al ,1978 , p . I-11 ) The Air Force called its defoliation activities in Vietnam ``Operation Ranch Hand (Gough , 1986...











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