The further the Vietnam War recedes into the past, the more preposterous it becomes. How could Americans have allowed President Lyndon Johnson to drag the United States into such a needless and futile struggle? Sending hundreds of thousands of US troops to fight in Southeast Asia turned out to be a monumental blunder. Was there no one in a position of influence or authority who could see that at the time? Where were the voices of sanity and reason?

Fifty years ago this month, in August 1964, Johnson offered the sane and reasonable a chance to make their case. What followed was a stupefying demonstration of groupthink. The guardians of conventional wisdom in the United States — its leading public officials and its major news outlets — all but automatically accepted the premise that the United States could, and should, determine the course of events in faraway Vietnam.

Citing alleged North Vietnamese attacks on US warships on Aug. 2 and 4, the president had requested congressional authorization “to take all necessary steps, including the use of armed force” to defend our South Vietnamese ally and thereby “prevent further aggression.” Unpack the language and Johnson was in effect asking Congress to declare war.

In what became known as the Tonkin Gulf Resolution, the Congress promptly complied. In the House of Representatives, the vote was 416-0. In the Senate, it was 88 to 2, with Alaska’s Ernest Gruening and Oregon’s Wayne Morse, both of the president’s own party, the sole dissenters. Some of those voting aye had their doubts, but these they duly suppressed. As Senator George Aiken, Republican of Vermont, put it, “As a citizen, I feel I must support our president whether his decision is right or wrong.” This was especially true when it came to standing up to communism.

The nation’s leading newspapers concurred with Aiken. The Tonkin Gulf Resolution found great favor with the Washington Post, which complimented legislators for “responding with commendable promptness and an almost unanimous voice.” Notwithstanding, “the reckless and querulous dissent of Senator Morse,” the overall effect was “to demonstrate before the world the unity of the American people in resisting Communist aggression.”

The Boston Globe likewise framed the issue in terms of justifiable defense. “Like a blackmailer, an aggressor will keep seeking more if he finds his crime brings benefits,” a Globe editorial observed. “Only when aggression is challenged can it be leashed, and, in future, deterred.” As the Globe saw it, ever since World War II the United States had been “painfully trying to indoctrinate the world with these elementary facts.” The Tonkin Gulf Resolution reaffirmed this ongoing American effort to educate the obdurate.

The New York Times shared this assessment. According to a Times editorial, President Johnson had “demonstrated his own capacity for toughness. And the Communists have been left in no doubt about American determination.” Toughness and determination, the Times believed, positioned the United States to bring peace to Vietnam.

Given the horrors that ensued, how can we explain this misplaced docility? On the Senate floor, Morse declared — accurately — that “we have been making covert war in Southeast Asia for some time instead of seeking to keep the peace.” Yet most members of Congress and newspaper editorial boards alike accepted at face value the Johnson administration’s version of what had happened in the Tonkin Gulf.

Similarly, they lazily concurred in the reflexive tendency to see events in Vietnam as the product of a monolithic Communist conspiracy. In fact, the monolith — if it ever existed — had already succumbed to the Sino-Soviet dispute. Yet acknowledging the existence of that dispute would have made it necessary to rethink the entire Cold War.

It takes gumption to question truths that everyone “knows” to be true. In the summer of 1964, gumption was in short supply. As a direct consequence, 58,000 Americans died, along with a vastly larger number of Vietnamese, Cambodians, and Laotians.

After 9/11, similar mistakes — deference to the official line and to the conventional wisdom (“terrorism” standing in for communism) — recurred, this time with even less justification. The misbegotten Iraq war was one result. Yet even today, events in Syria, Ukraine, and elsewhere elicit an urge to “do something,” accompanied by the conviction that unless troops are moving or bombs dropping the United States is somehow evading its assigned responsibilities. The question must be asked: Are Americans incapable of learning?