What was the main reason that the united states became involved in the vietnam conflict? a. the united states wanted to support france in its fight to retain its colonies in southeast asia. b. the united states had established an alliance with the government of north vietnam. c. the united states wanted to strengthen the viet minh its struggle against china. d. the united states worried that if vietnam fell to the communists, other countries in southeast asia would follow.

Correct D. The  United States worried that if Vietnam fell to the communists, other countries in Southeast Asia would follow.

This was known as "domino theory" -- that the spread of communism in Vietnam would result in other countries in the region falling like dominoes to the influence of communism.  

Domino theory was, in a sense, a corollary to the policy of containment that had been the United States' goal regarding communism since the end of World War II.  George F. Kennan is known for recommending the policy of containment.  Kennan was an American diplomat in Moscow who, in 1946,  sent a long telegram of his advice about what the USA needed to do about the Union of Soviet Socialist Republics (USSR). Kennan recommended containing the communist influence the USSR and its allies had.  So whenever there was a threat of the spread of communism, as there was in Vietnam, the US tended to react to try to stop that spread.