Brandishing a captured Chinese machine gun, Secretary of Defense Robert S. McNamara appeared at a televised news conference in the spring of 1965. The United States had just sent its first combat troops to South Vietnam, and the new push, he boasted, was further wearing down the beleaguered Vietcong.

“In the past four and one-half years, the Vietcong, the Communists, have lost 89,000 men,” he said. “You can see the heavy drain.”

That was a lie. From confidential reports, McNamara knew the situation was “bad and deteriorating” in the South. “The VC have the initiative,” the information said. “Defeatism is gaining among the rural population, somewhat in the cities, and even among the soldiers.”

Lies like McNamara’s were the rule, not the exception, throughout America’s involvement in Vietnam. The lies were repeated to the public, to Congress, in closed-door hearings, in speeches and to the press. The real story might have remained unknown if, in 1967, McNamara had not commissioned a secret history based on classified documents — which came to be known as the Pentagon Papers.

By then, he knew that even with nearly 500,000 U.S. troops in theater, the war was at a stalemate. He created a research team to assemble and analyze Defense Department decision-making dating back to 1945. This was either quixotic or arrogant. As secretary of defense under Presidents John F. Kennedy and Lyndon B. Johnson, McNamara was an architect of the war and implicated in the lies that were the bedrock of U.S. policy.

Daniel Ellsberg, an analyst on the study, eventually leaked portions of the report to The New York Times, which published excerpts in 1971. The revelations in the Pentagon Papers infuriated a country sick of the war, the body bags of young Americans, the photographs of Vietnamese civilians fleeing U.S. air attacks and the endless protests and counterprotests that were dividing the country as nothing had since the Civil War.

The lies revealed in the papers were of a generational scale, and, for much of the American public, this grand deception seeded a suspicion of government that is even more widespread today.

Officially titled “Report of the Office of the Secretary of Defense Vietnam Task Force,” the papers filled 47 volumes, covering the administrations of President Franklin D. Roosevelt to President Lyndon B. Johnson. Their 7,000 pages chronicled, in cold, bureaucratic language, how the United States got itself mired in a long, costly war in a small Southeast Asian country of questionable strategic importance.

They are an essential record of the first war the United States lost. For modern historians, they foreshadow the mind-set and miscalculations that led the United States to fight the “forever wars” of Iraq and Afghanistan.

The original sin was the decision to support the French rulers in Vietnam. President Harry S. Truman subsidized their effort to take back their Indochina colonies. The Vietnamese nationalists were winning their fight for independence under the leadership of Ho Chi Minh, a Communist. Ho had worked with the United States against Japan in World War II, but, in the Cold War, Washington recast him as the stalking horse for Soviet expansionism.

American intelligence officers in the field said that was not the case, that they had found no evidence of a Soviet plot to take over Vietnam, much less Southeast Asia. As one State Department memo put it, “If there is a Moscow-directed conspiracy in Southeast Asia, Indochina is an anomaly.”

But with an eye on China, where the Communist Mao Zedong had won the civil war, President Dwight D. Eisenhower said defeating Vietnam’s Communists was essential “to block further Communist expansion in Asia.” If Vietnam became Communist, then the countries of Southeast Asia would fall like dominoes.

This belief in this domino theory was so strong that the United States broke with its European allies and refused to sign the 1954 Geneva Accords ending the French war. Instead, the United States continued the fight, giving full backing to Ngo Dinh Diem, the autocratic, anti-Communist leader of South Vietnam. Gen. J. Lawton Collins wrote from Vietnam, warning Eisenhower that Diem was an unpopular and incapable leader and should be replaced. If he was not, Gen. Collins wrote, “I recommend re-evaluation of our plans for assisting Southeast Asia.”

Secretary of State John Foster Dulles disagreed, writing in a cable included in the Pentagon Papers, “We have no other choice but continue our aid to Vietnam and support of Diem.”

Nine years and billions of American dollars later, Diem was still in power, and it fell to President Kennedy to solve the long-predicted problem.

After facing down the Soviet Union in the Berlin crisis, Kennedy wanted to avoid any sign of Cold War fatigue and easily accepted McNamara’s counsel to deepen the U.S. commitment to Saigon. The secretary of defense wrote in one report, “The loss of South Vietnam would make pointless any further discussion about the importance of Southeast Asia to the Free World.”

The president increased U.S. military advisers tenfold and introduced helicopter missions. In return for the support, Kennedy wanted Diem to make democratic reforms. Diem refused.

A popular uprising in South Vietnam, led by Buddhist clerics, followed. Fearful of losing power as well, South Vietnamese generals secretly received American approval to overthrow Diem. Despite official denials, U.S. officials were deeply involved.

“Beginning in August of 1963, we variously authorized, sanctioned and encouraged the coup efforts …,” the Pentagon Papers revealed. “We maintained clandestine contact with them throughout the planning and execution of the coup and sought to review their operational plans.”

The coup ended with Diem’s killing and a deepening of American involvement in the war. As the authors of the papers concluded, “Our complicity in his overthrow heightened our responsibilities and our commitment.”

Three weeks later, President Kennedy was assassinated, and the Vietnam issue fell to President Johnson.

He had officials secretly draft a resolution for Congress to grant him the authority to fight in Vietnam without officially declaring war.

Missing was a pretext, a small-bore “Pearl Harbor” moment. That came on Aug. 4, 1964, when the White House announced that the North Vietnamese had attacked the U.S.S. Maddox in international waters in the Gulf of Tonkin. This “attack,” though, was anything but unprovoked aggression. Gen. William C. Westmoreland, the head of U.S. forces in Vietnam, had commanded the South Vietnamese military while they staged clandestine raids on North Vietnamese islands. North Vietnamese PT boats fought back and had “mistaken Maddox for a South Vietnamese escort vessel,” according to a report. (Later investigations showed the attack never happened.)

Testifying before the Senate, McNamara lied, denying any American involvement in the Tonkin Gulf attacks: “Our Navy played absolutely no part in, was not associated with, was not aware of any South Vietnamese actions, if there were any.”

Three days after the announcement of the “incident,” the administration persuaded Congress to pass the Tonkin Gulf Resolution to approve and support “the determination of the president, as commander in chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression” — an expansion of the presidential power to wage war that is still used regularly. Johnson won the 1964 election in a landslide.

Seven months later, he sent combat troops to Vietnam without declaring war, a decision clad in lies. The initial deployment of 20,000 troops was described as “military support forces” under a “change of mission” to “permit their more active use” in Vietnam. Nothing new.

As the Pentagon Papers later showed, the Defense Department also revised its war aims: “70 percent to avoid a humiliating U.S. defeat … 20 percent to keep South Vietnam (and then adjacent) territory from Chinese hands, 10 percent to permit the people of South Vietnam to enjoy a better, freer way of life.”

Westmoreland considered the initial troop deployment a stopgap measure and requested 100,000 more. McNamara agreed. On July 20, 1965, he wrote in a memo that even though “the U.S. killed-in-action might be in the vicinity of 500 a month by the end of the year,” the general’s overall strategy was “likely to bring about a success in Vietnam.”

As the Pentagon Papers later put it, “Never again while he was secretary of defense would McNamara make so optimistic a statement about Vietnam — except in public.”

Fully disillusioned at last, McNamara argued in a 1967 memo to the president that more of the same — more troops, more bombing — would not win the war. In an about-face, he suggested that the United States declare victory and slowly withdraw.

And in a rare acknowledgment of the suffering of the Vietnamese people, he wrote, “The picture of the world’s greatest superpower killing or seriously injuring 1,000 noncombatants a week, while trying to pound a tiny backward nation into submission on an issue whose merits are hotly disputed, is not a pretty one.”

Johnson was furious and soon approved increasing the U.S. troop commitment to nearly 550,000. By year’s end, he had forced McNamara to resign, but the defense secretary had already commissioned the Pentagon Papers.

In 1968, Johnson announced that he would not run for re-election; Vietnam had become his Waterloo. Nixon won the White House on the promise to bring peace to Vietnam. Instead, he expanded the war by invading Cambodia, which convinced Daniel Ellsberg that he had to leak the secret history.

After The New York Times began publishing the Pentagon Papers on Sunday, June 13, 1971, the nation was stunned. The response ranged from horror to anger to disbelief. There was furor over the betrayal of national secrets. Opponents of the war felt vindicated. Veterans, especially those who had served multiple tours in Vietnam, were pained to discover that Americans officials knew the war had been a failed proposition nearly from the beginning.

Convinced that Ellsberg posed a threat to Nixon’s re-election campaign, the White House approved an illegal break-in at the Beverly Hills, Calif., office of Ellsberg’s psychiatrist, hoping to find embarrassing confessions on file. The burglars — known as the Plumbers — found nothing, and got away undetected. The following June, when another such crew broke into the Democratic National Committee Headquarters in the Watergate complex in Washington, they were caught.

The North Vietnamese mounted a final offensive, captured Saigon and won the war in April 1975. Three years later, Vietnam invaded Cambodia — another Communist country — and overthrew the genocidal Khmer Rouge regime. That was the sole country Communist Vietnam ever invaded, forever undercutting the domino theory — the war’s foundational lie.