How did the domino theory cause the Vietnam war?

The domino theory was a foreign policy theory during the 1950s to 1980s, promoted at times by the government of the United States, that speculated that if one land in a region came under the influence of communism, then the surrounding countries would follow in a domino effect. From the late 1950s to 1975, the United States fought in its only war that did not end in a U.S. victory. During this war, the actions that fulfilled the policies of the United States changed often over the course of four presidents and about eighteen years.
America started to slide down a slippery slope of military involvement - it launched the MAAG: Military Assistance Advisory Group. Then, in 1953, Dwight D. Eisenhower was inaugurated as President. As he sends more and more monetary aid to the French, he coined the phrase "domino effect." Simply put, the domino effect stated that if one Southeast Asian country fell to communism, then all of Southeast Asia would fall. And to that end, Eisenhower said, "…The possible consequences of the loss of free countries in Southeast Asia are just incalculable to the free world." Many succeeding presidents used this term to justify their increasing involvement in Southeast Asia.