Answer:
The Vietnam War
Explanation:
Perhaps the most famous American proxy war is the Vietnam war, because it was very long (it lasted 19 years, from 1955 to 1975), very deadly (over 1 million people died in total, the majority, Vietnamese), and specially, because the United States lost against a rival, the Vietcong, which was supposed to be weaker.
The American leadership at the time believed that it was important to fight tthe war in Vietnam because of the Domino theory effect, a theory formulated by former president Dwight Eisenhower.
Eisenhower believed that if a country like Vietnam fell to communism, its neighbors would follow suit: Laos, Cambodia, Thailand, and even Indonesia would quickly become communist countries according to this theory.
To prevent this effect, the war was launched, in order to defeat the Vietnamese communist. The war was a failure, and Vietnam is a communist country (at least in paper) up to this day.
