The Vietnam War lasted from 1959 to 1975, and US involvement increased in the early 1960s. During the Vietnam Era there was a movement to allow 18 year olds to legally drink alcohol. Proponents of an 18 year old minimum drinking age argued that soldiers who were legally adults and were dying for their country were old enough to legally drink alcohol (many people today still argue this should be the case).
Many states changed their own laws to allow 18 years old to legally drink alcohol during the Vietnam Era. Most military bases, even those in states with 21 minimum drinking ages, would allow active military personnel, while on base, to consume alcoholic beverages at age 18.
Some states allowed people from age 18 to under 21 to drink "three-two" alcoholic beverages. "Three-two" stood for the percentage of alcohol in the beverage. For example, a "three-two" beer was 3.2% alcohol.
Next, we’ll see why every state has raised its minimum drinking age to 21 years old.