Their numbers surpassed 16,000 by the time of his assassination.

Following Kennedy's death, President Lyndon Johnson retained McNamara as "the best in the lot" of Kennedy Cabinet members and the man to keep Vietnam from falling to the Communists.

When U.S. naval vessels were allegedly attacked off the North Vietnamese coast in 1964, McNamara lobbied Congress to pass the Tonkin Gulf Resolution, which Johnson used as the equivalent of a congressional declaration of war.

McNamara visited Vietnam — the first of many trips — and returned predicting that American intervention would enable the South Vietnamese, despite internal feuds, to stand by themselves "by the end of 1965."

That was an early forerunner of a seemingly endless string of official "light at the end of the tunnel" predictions of American success. Each was followed by more warfare, more American troops, more American casualties, more American bombing, more North Vietnamese infiltration — and more predictions of an early end to America's commitment.

As the years passed, the war became increasingly controversial. Among those who marched protest was a young American attending Oxford University, Bill Clinton. Another protester, in California, was Craig McNamara, a teenager when his father ran the war.

In 1984, in an interview with Paul Hendrickson of the Washington Post, Craig recalled how McNamara would not talk about Vietnam for years afterward.

"Nobody can get anywhere on Vietnam with my father, including me," Craig said. "It's just not in his scope to communicate his deepest thoughts and feelings to me."

Toward the end, McNamara found himself pitted against the Joint Chiefs of Staff, who wanted unremitting and wide-ranging bombing of the North.

He became openly skeptical about the effectiveness of bombing the north to cut down the infiltration of men and war supplies to the south. At McNamara's request, Johnson halted the bombing in December 1965 to induce North Vietnam to enter into peace negotiations. Nothing happened and Johnson resumed the bombing at the end of January.

McNamara, with Paul Warnke and Paul Nitze, privately transmitted a peace proposal to the North Vietnamese in August 1967. It was rejected in October. With 1,000 Americans now dying each month, McNamara recommended a bombing halt, a freeze in U.S. troop levels and a turnover of war responsibility to Saigon; Johnson rejected the idea.

The president lost faith in his secretary. McNamara would later write that he didn't know if he quit or was fired.

At a Feb. 29, 1968, retirement ceremony, he was overcome with emotion and could not speak. Johnson put an arm around his shoulder and led him from the room.

McNamara's first wife, Margaret, whom he met in college, died of cancer in 1981; they had two daughters and a son. In 2004, at age 88, he married Italian-born widow Diana Masieri Byfield.

—Managing News Editor Lara Zade contributed to this report.