There are many reasons why Robert McNamara ceased to be secretary of defence.
Robert McNamara became secretary of defence during Kennedy's presidency in 1961. Johnson had always thought very highly of McNamara. McNamara had been vital in the formulation of Kennedy and Johnson’s Vietnam policies. However, bobby Kennedy had become passionately anti-war and publicly opposed the war from January 1966. Johnson privately insulted ‘Nervous Nellies’ who disagreed with him about Vietnam, calling one senator a ‘prick’, Senator Fulbright ‘Senator Halfbright’ and Bobby Kennedy ‘spineless’. Johnson considered the Kennedy-McNamara friendship dangerous.
One reason why McNamara ceased to be secretary of defence is, that he wanted to decrease the fighting for the U.S and hand over the conflict to South Vietnam. In an early November 1967 memo to Johnson, McNamara's recommendation to freeze troop levels, stop bombing North Vietnam and for the U.S. to hand over ground fighting to South Vietnam was rejected outright by the President. McNamara's recommendations amounted to his saying that the strategy of the United States in Vietnam which had been pursued to date had failed. McNamara later stated he "never heard back" from Johnson regarding the memo.
Another reason for his resignation is that, McNamara’s health and family life suffered because of the Vietnam War. His children had told him that what the U.S was doing was not right. They participated in the anti-war movement and protests. McNamara blamed himself for his wife’s stomach ulcer. McNamara seemed physically and mentally exhausted. He began to have severe chest pains.
Largely as a result, on 29th November 1967, McNamara was relieved to announce his pending resignation and that he would become President of the World Bank. He was replaced by Clark Clifford in January 1968. He had not advocated the U.S leaving Vietnam, but he had stopped the escalation for the time being, an option which his colleagues saw as unacceptable....