Which chemical defoliant was widely used by the US military in Vietnam?

The chemical defoliant widely used by the US military in Vietnam was Agent Orange. It was designed to strip leaves from dense forests and destroy crops, making it harder for North Vietnamese forces and the Viet Cong to hide, move, or obtain food.

Agent Orange was used mainly during Operation Ranch Hand, the US herbicide campaign that ran from 1962 to 1971. It was one of several so-called Rainbow Herbicides, whose names came from colored identification bands on their storage drums. Agent Orange was a mixture of 2,4-D and 2,4,5-T, and some batches contained the highly toxic dioxin TCDD as a contaminant.

The name can be misleading: Agent Orange was not an orange-colored gas, and it was not a conventional chemical weapon such as mustard gas. It was sprayed from aircraft, helicopters, boats, trucks, and other equipment. The defoliation campaign caused severe and lasting environmental damage, while exposure became associated with serious health effects among Vietnamese civilians and military veterans.

The US sprayed several herbicides in Vietnam, but Agent Orange became the best-known because of the scale of its use and the long-term controversy surrounding dioxin contamination.