Truman doctrine and "Domino Theory" regarding the containment of communism is central to any summation for the causes of the Vietnam War. Truman Doctrine is the US policy to "contain" Communism and not allow it to spread to the third world. The "Domino Theory" is a continuation of the doctrine which theorizes that the conversion of a nation to Communism will catalyze the spread of Communism further in that region. US involvement in Vietnam was the result of a foreign policy dictated by these two core principals.

A little back-drop: I wrote my thesis on the Vietnam War, and the best book that I came across is Robert D. Schulzinger's "A Time for War: The United States and Vietnam 1941-1975." I still own the book to this day.

Everything, that hawailann says is true, but I would like to add a little nuance to his or her general explanation. The Domino Theory originated in the 1950s and was publicly articulated by Eisenhower in 1954 in contemplation of the spread of communism in SE Asia, though Eisenhower did not "create" the Domino Theory by any means. (Source: Schulzinger's book & the wiki page on Domino Theory). Additionally, the policy of "containment" began in the aftermath of WWII, and the policies are inherently related though not interchangeable.

A big thing that was not mentioned is that the Domino Theory really relied on the assumption that communism was monolithic, with orders coming from Moscow. In the case of Vietnam, at least, this was not the case; the Communist in Vietnam were really nationalists first and foremost, fighting first against French Imperialism in the 50s and then perceived American Imperialism a decade later. In fact, Ho Chi Minh had ties closer to China than the Soviet Union. (Source: Schulzinger's book). Despite the later realized fact that communism in SE Asia had a strong nationalist aspect to it, American policymakers buying in to the Domino Theory viewed each country in limbo of turning communist as a potential dependent Soviet Satellite State. Accordingly, even losing a relatively small country's fall to communism could lead to the potential expansion of communism into the entire region.

The policy of containment and the Domino Theory explains the ostensible American "interests" in Vietnam; however, in order to understand why those "interests" overshadowed other considerations, you have to understand who was influencing policy at the Whitehouse. Robert McNamara was Secretary of Defense from 1961-1968 and Dean Rusk was Secretary of State from 1961-1969. They were both old-school Establishment guys, 1950s-style Domino Theory adherents and they strongly urged President Kennedy to get involved in Vietnam, despite Kennedy's initial misgivings. (Source: Schulzinger, 111-113). It is important that the "aristocratic," Harvard-educated Kennedy was relatively self-assured when it came to foreign affairs, so he was more inclined to second guess his sometimes hawkish foreign policy advisors. In the end, Kennedy supported the Domino Theory and did put a few American troops in Vietnam to advise the South Vietnamese, as well as a few helicopters and pilots. (Source: http://www.historylearningsite.co.uk/kennedy_vietnam.htm).

By comparison, LBJ's self-percieved strengths were not in the realm of foreign affairs. A self-made man, LBJ came from poverty and attended Southwest Texas State Teacher's College. (Robert Caro has written a few AWESOME and meticulously researched biographies on LBJ.) LBJ rose through the legislature and possessed tremendous abilities when it came to domestic affairs. In fact, during his presidency, he viewed the escalating war as a distraction that threatened his extensive domestic agenda. However, LBJ felt out of his depth when it came to foreign policy. Throughout the war, he deferred heavily to McNamara and Rusk (Kennedy's men…. men he choose not replace); but perhaps just as importantly, he relied largely on a group of men (known as Johnson's "Wise Men") who were all ivy-educated members of the 1950s "Establishment" that still considered the Domino Theory to be dogma. So where Kennedy stuck his toe in the water, Johnson allowed his advisors to lead him off the diving board and do a cannonball. (Just a little something to get you up to speed on these guys http://en.wikipedia.org/wiki/The_Wise_Men_(book)).

Johnson was never truly comfortable with the war in Vietnam, but he continued escalation because those to whom he deferred consistently told him that: a) the war was winnable, and b) the war had to be won. Additionally, as domestic protest of the war started to become more rampant, it seemed as though withdrawal would not only be perceived as a win for the communist, it would also be a win for the protesting, anti-Establishment domestic population. It was only in the aftermath of the Tet Offensive that the Wise Men realized that the war was not winnable under the desired cost parameters (in terms of troops, resources, and political capital) and suggested withdrawal to Johnson in March of 1968. To be fair to the Wise Old Men, their opinions were formed on the basis of military intelligence which tended, pre-Tet, to have an optimistic sheen to it. Not surprisingly, Johnson listened to his Wise Old Men. Of course, he did not run in 1968, so withdrawal would not occur for another half a decade.

Note: In his book, Schulzinger does not place as much emphasis as I do on the influence that the Wise Old Men had on Johnson; however, the general point remains the same…. Johnson deferred to people he considered foreign policy and military experts on Vietnam (for our purposes, it is not terribly important to distinguish whether we are talking about his Cabinet or military personnel, like the Joint Chiefs of Staff, or his unofficial advisors, like the Wise Old Men), and to those experts, the need to halt the expansion of communism was the key American "interest" in Vietnam that justified putting, and keeping, boots on the ground.