A telltale sign that Defense Secretary Robert S. McNamara no longer believed in the Vietnam War came not from anything he said publicly, but how he said it.

Harold Brown, who served under McNamara at the Pentagon as director of defense research and engineering and later as secretary of the Air Force, came to recognize the mannerisms McNamara would display when voicing public support for policies with which he privately disagreed.

McNamara would “lean forward and pull his socks up,” Brown said Tuesday.

“Bob became skeptical about the Vietnam War well before he stopped talking about how great it was going,” added Brown, who later served as secretary of defense under President Jimmy Carter.

But McNamara was “extremely loyal” to President Lyndon B. Johnson, Brown added, and refused to publicly disagree with LBJ’s Vietnam policy.

Brown was part of a panel Tuesday at the National Archives discussing a new history of McNamara’s tenure at the Pentagon, published by the Historical Office of the Secretary of Defense.

“This is not your standard, bland official history,” George C. Herring, one of the nation’s leading scholars on the Vietnam War, told the audience at the panel discussion.

“McNamara, Clifford and the Burdens of Vietnam, 1965-1969,” written by Edward J. Drea, covers the final three years of Mc­Namara’s tenure and the one-year term of Clark M. Clifford, who took the office after Mc­Namara’s resignation in 1968.

“[F]or all luminous achievements, his choices that led to the Vietnam disaster will forever remain McNamara’s enduring legacy,” Drea concludes in the book.

An earlier volume, “The McNamara Ascendancy, 1961-1965,” published in 2006 and co-written by Drea, covered McNamara’s first four years in office, including his time under President John F. Kennedy.

Working for the Office of the Secretary of Defense, Drea, a military historian who served in Vietnam, had access to papers that remain classified.

Herring called the new book “a major contribution to our understanding of one of the most tumultuous periods in our history.”

Although the book covers many aspects of the Defense Department during the latter half of the 1960s, McNamara dominates the story, from Vietnam to his efforts to tame the Pentagon’s bureaucracy. “[N]o one since has approached his mastery of the enormity and complexity of the Pentagon,” Drea writes.

Drea’s work examines Mc­Namara’s and Clifford’s handling of the war in the broader context of tumultuous times around the globe, including the Soviet invasion of Czechoslovakia, the 1967 war in the Middle East and the North Korean seizure of the USS Pueblo.

After Clifford took office, he was determined to end a war he believed was a disaster. “Clifford saw his major role as convincing LBJ to get out of Vietnam,” Drea told the audience Tuesday. Consequently, Drea added, Clifford left almost all the day-to-day operations of the Pentagon to the deputy secretary, Paul Nitze.

Brown said that one lesson from McNamara’s tenure is that four years is long enough to serve as secretary of defense. In Washington, Brown noted, “friends come and go, but enemies accumulate.”

McNamara was “almost unprecedently successful in his first four years, and the last three were a tragedy,” Brown said.