How does the domino theory explain American involvement in southeast Asia?

To contain the Communism throughout Indochina, for America had a theory if one nation falls to Communism, than rest of the neighboring nations will fall and eventually make it to the United States soil, as they were afraid. Soviet Nation V.s United States of Democracy were huge rivals during the Cold War, it was all about competition and the fear that one day each other death squads might obliterate each others land and cause massive genocide between each other.