Us entered Vietnam war in order to stop the spread of communism. It was justified as the whole nation was trying to follow the unrealistic principles of communism and especially Asia gave enormous support to Soviet Russia in spread of communism.
Step-by-step explanation:
The Domino theory began in 1950s wherein it was feared that one Asian country when fallen as a victim to communism then it would spread its dominance over all other Asian countries and in order to strop that, US entered the Vietnam war.
Also there was red scare that prevailed in America, Mc Carthy' statement created fear among the people that communism may have its secret roots spread in America too. So these factors made US to enter Vietnam war. The conflict continued for a decade. Many millions of American served in Vietnam and out of which many in thousands lost their lives.