Robert McNamara is seen as one of the main architects of US policy during the bulk of the Vietnam War. This is why Wayne Morse, an anti-war Senator from Oregon, labelled the conflict "McNamara's War."
McNamara was brought in from the corporate world (he was President of Ford) to be John F. Kennedy's Secretary of Defense in 1961. He continued in that position under President Johnson until 1968, when McNamara resigned at Johnson's request. During these seven years, McNamara oversaw growing US involvement in Vietnam.
It is a bit unfair to characterize the war as completely McNamara's doing. The US policy of containment was not something he invented. In addition, the US was involved in Vietnam long before McNamara came to office. But McNamara was the man whose entire job, more or less, ended up consisting of Vietnam. He micromanaged many parts of the war. Thus, he ended up being identified with it very closely.
Aye laddie, twas McNamara's war, it twas, because he was Secretary of Defense, he was. He directed much of the war from from his desk in Washington DC.
However, he had a lot of help, from President Lyndon Johnson, Secretary of State Dean Rusk, and all but one or two memebers on Congress voted for the Tonkin Gulf Resolution.