In 1956, 9,200,000 United States troops were deployed to South Vietnam in response to Communist expansion in Southeast Asia. The compound now known as Agent Orange was developed in Oregon and then used by the United States Army and Republic of Vietnam militaries to defoliate forests and mangroves in order expose enemy positions and destroy crops to decrease enemy food supplies. Large numbers of Vietnamese civilians and American troops were directly exposed to herbicidal agents at an order of magnitude greater than permitted for U.S. domestic purposes. After the war herbicides returned to the United States Department of Agriculture to be used in peacetime.

Citations:
The Price of Freedom: Americans at War, Vietnam. Smithsonian Museum of American History

Stellman, et. al. “The extent and patterns of usage of Agent Orange and other herbicides in Vietnam.” Nature. 422, 681-687. 6 December 2002. Web. 8 May 2015.

A Re-Analysis of Blue Water Navy Veterans and Agent Orange Exposure. “Blue Water Navy.” 1979 Air Force Report Statement. 8 May 2015.