President Kennedy appointed McNamara Defense Secretary in 1961, position that he occupied until 1968, when Johnson replaced him with Clark Clifford. McNamara had an enormous influence on the U.S. policy towards Vietnam. During his Kennedy years, he favored a hard-line approach to the conflict in Vietnam, and already in 1961 he advocated a sizable deployment of U.S. combat troops.

As the war went on without apparent progress, McNamara grew skeptical of the effectiveness of the military action. In November 1965, he doubted that even a force of 600,000 troops could guarantee success. By 1966, he was convinced of the futility of the bombing campaign in North Vietnam. At the time of his resignation, he was deeply disappointed with the war and with his personal role in it.

Biography of Robert McNamara
From The U.S. Defense Department
http://www.defenselink.mil/specials/secdef_histories/bios/mcnamara.htm

Draft Memorandum from McNamara to Kennedy, November 5, 1961
From the American War Library
http://members.aol.com/warlibrary/vwrsm5.htm

William Bundy drafted this memorandum for McNamara. In the document, the Defense Secretary assumes that a maximum of 220,000 men would suffice to accomplish U.S. objectives. McNamara advocates a firm commitment from the start and warns the President of the possible consequences on the domestic level of a policy of gradual involvement.

Excerpts from Rusk-McNamara Report to Kennedy, November 11, 1961
From the Vietnam Wars Page at Vassar College
http://vietnam.vassar.edu/~vietnam/doc7.html

After their assessment trip to Vietnam, Secretary of State Rusk and McNamara wrote this report for Kennedy. McNamara and Rusk recommended a boost to U.S. military presence "...required for increased United States participation in the direction and control of GVN military operations."

McNamara's advice to President Johnson
From Professor Sage's Page at Northern Virginia Community College
http://www.nvcc.edu/home/nvsageh/Hist122/Part4/McNamara.htm

1965 Memorandum from McNamara to Johnson where the Secretary of Defense outlines the steps to take in order to win the war. These steps included: increased U.S. presence in South Vietnam, military action against North Vietnam and political initiatives centered mainly in international players, more than the political reality of South Vietnam.

April 15, 1965, telegram to Ambassador Taylor on boosting U.S. military role in South Vietnam
From the Historian of the State Department
http://www.state.gov/www/about_state/history/vol_ii/241_260.html (Doc.#256)

The Pentagon instructs Taylor of the new, more aggressive involvement of U.S. troops in the field. They are to assist S. Vietnam troops in combat and counter-insurgency operations. On the same page (Doc. # 258), Taylor expresses his dismay at what he sees as a military takeover of the U.S. mission in South Vietnam.

McNamara's July 20, 1965 Memorandum to Johnson
From Mark Bradley's Page at the University of Milwaukee
http://www.uwm.edu/~mbradley/mcnamaradoc2.html

In this memorandum, McNamara argues that the war is evolving into a conventional conflict, in which U.S. support for South Vietnam would be most effective. Consequently, U.S. forces have to be increased, even at the risk of casualties. McNamara could not predict the casualty level, but estimates that "the U.S. killed-in-action might be in the vicinity of 500 a month by the end of the year."