In the Vietnam War, defoliants were dropped from American helicopters to try and remove foliage that guerrilla Viet Cong troops could possibly hide in. The most popular defoliant, named Agent Orange, caused many environmental problems and the leftover chemicals are still harming crops and civilians in Vietnam today.
According to the United States Department of Military Affairs, this chemical, Agent Orange, is associated with 14 diseases and cancers, including Parkinson's disease, a progressive disorder of the nervous system that affects muscle movement; respiratory cancers, such as cancers of the lung, larynx, trachea, and bronchus; and Non-Hodgkin's Lymphoma, a group of cancers that affect the lymph glands and other lymphatic tissue.
According to the article "'Last ghost' of the Vietnam War", Vietnam estimates that 400,000 people were killed or maimed by the defoliants, including Agent Orange; 500,000 children have been born with defects, such as mental retardation; and a further two million people have suffered cancers or other illnesses. In addition to health issues to humans, Agent Orange negatively affected the environment in Southeast Asia. To this day, defoliants dropped on Vietnam during the Vietnam War are still harming forests and crops.
Footnotes:
3: Header photo obtained from http://www.history.com/shows/vietnam-in-hd/photos/vietnam-war/photo14
3: Header photo obtained from http://www.history.com/shows/vietnam-in-hd/photos/vietnam-war/photo14