Though alcohol was the drug most commonly used by American troops in the Vietnam War, cannabis was the second-most common. Initially rates of usage among deployed soldiers were comparable to those of their stateside peers, with 29% of troops departing Vietnam in 1967 reporting having ever used marijuana in their lives. A 1976 study however showed that from 1967–1971, the proportion of troops having used marijuana peaked at 34% before stabilizing to 18%, while the number of troops who had used cannabis prior to deployment stayed around 8%.[14] Cannabis use by troops in Vietnam was generally dealt with using Article 15 non-judicial punishment in units such as the 101st Airborne.[15]
During the Vietnam War period, cannabis use also became common among US forces in the United States and in Europe, with a 1971 article claiming that over 1,000 midshipmen at Annapolis Naval Academy used cannabis, and a survey in Germany showing that half of the soldiers in the surveyed battalion were regular cannabis users.[16]
https://en.wikipedia.org/wiki/Cannabis_and_the_United_States_military