Answer:
They did not want to lose South Vietnam to communism, is the right answer.
Explanation:
During the decades between the 1950's to the 1980's (Cold War), the domino theory was a policy that implied that if a country in a region comes under the sway of communism, then the other neighboring countries of the will fall in communism. This theory suggested that a communist government in a nation can help the communists to take over the other countries of the region. By opting this theory, the United States justified its involvement in the Vietnam War and its support for the dictator of South Vietnam who was Non-Communist.