Articulating the main reason for U.S. involvement in Vietnam is a little more complicated than it may seem. The reason for this was the somewhat haphazard manner in which the United States allowed itself to become mired in the conflicts in Southeast Asia centered primarily on Vietnam. The short answer to the question is that the United States feared the spread of communism across Asia and viewed Vietnam as an important place at which to halt that spread. The more protracted answer, however, required discussion of the French experience in what was then referred to as Indochina. France had colonized much of Southeast Asia (Vietnam, Cambodia, Laos), and following Japan's defeat in World War II -- Japan had occupied the region during the war, evicting the French -- France sought to regain its colonial holdings in Indochina. The Vietnamese nationalists and communists, under the leadership of Ho Chi Minh, however, were keen to prevent the French from reoccupying their country. Newly-liberated from the Japanese, the Vietnamese were hopeful of establishing an independent nation, albeit one the ideological orientation of which remained more than a little murky, Ho and his followers being devout Marxists. The war that emerged between Ho's communist militia, the Viet Minh, and the French provided the seeds of the American involvement.
The Viet Minh succeeded in militarily defeating French forces at the battle of Dien Bien Phu in 1954. Prior to this defeat, however, the French government had essentially blackmailed the United States into supporting France's efforts at reclaiming its Southeast Asian colonies. The United States was desperate for French cooperation in the establishment of the North Atlantic Treaty Organization (NATO), the alliance developed to defend Western Europe against the Soviet Union. The French leveraged that American desperation to secure U.S. help in reestablishing its colonial holdings in Southeast Asia. After the French defeat at Dien Bien Phu, the United States then found itself left 'holding the bag,' so to speak. Ho Chi Minh's Marxist orientation, and a growing U.S. commitment to prevent Ho's forces from conquering the southern half of the country and establishing a communist beachhead that would spread across the region, resulted in the escalating U.S. role there.
During the 1950s, when debates regarding Southeast Asia were carried out in the halls of Congress, there was a great deal of sympathy for the notion of supporting an independent, pro-Western South Vietnam. Fears of Soviet and Chinese-inspired communist expansionism were prevalent throughout much of the United States, including among the nation's elected representatives, including a young Massachusetts senator named John Kennedy who argued valiantly for a stronger U.S. response to the communist threat in Vietnam. The need to confront Soviet and Chinese-sponsored insurgencies in what was called "the Third World" was widely shared among the country's elected representatives. The war that would develop would, by the middle of the 1960s, see that pro-war consensus shatter, as demonstrations broke out across the United States and television broadcast a steady flow of dead and maimed American soldiers that eroded public support for the military efforts in Vietnam.
The main reason for the U.S. involvement in Vietnam, in conclusion, was the perceived need to contain communism from spreading across Asia. The actual U.S. involvement there, however, owed a great deal to France's desire to reclaim its old colonies.