Why did the US continue to fight in Vietnam?

. Although Vietnam is a poor Asian country, the people of the United States fought there in order to stop the domino effect of the communists. This theory stated, If South Vietnam falls to the Communist, Laos, Cambodia, Thailand, Burma, India and Pakistan would also fall like dominos. The Pacific Islands and even Australia could be at risk. Basically the war was fought to hold the line against the spread of world Communism. America paid for the war the French fought against Communist Vietnam as a part of the Truman Doctrine and then by the 1950’s became involved when the war flared up again, the war against communism that is.