Spread of communism (The Domino Theory) and its support from China and Russia.
Explanation:
Before the Vietnam war, the United States was very much concern about the spread of Communism in Asia, as they gave it a term of Domino theory. The theory asserted the situation of letting a communist nation to takeovers other neighbouring states and converting it under communism. The U.S. involvement in Vietnam began in 1954, though ongoing conflict in the region of North and South.
Russia assisted China in spreading communism in Asia which also became one of the reasons for the United States to refuse to withdraw from the Vietnam conflict.
