Fifty years ago today, the national police chief of South Vietnam calmly approached a prisoner in the middle of a Saigon street and fired a bullet into his head.

A few feet away stood Eddie Adams, an Associated Press photographer, eye to his viewfinder. On a little piece of black-and-white film, he captured the exact moment of the gunshot.

The police chief, Brig. Gen. Nguyen Ngoc Loan, stands with his back to the camera, right arm fully extended, left arm loosely by his side. The prisoner, Nguyen Van Lem, is a Vietcong fighter but wears no uniform, only a plaid shirt and black shorts. His hands are cuffed behind his back. Though in his 30s, he looks little older than a boy. His face is contorted from the bullet’s impact.

By morning, this last instant of his life would be immortalized on the front pages of newspapers nationwide, including The New York Times. Along with NBC film footage, the image gave Americans a stark glimpse of the brutality of the Vietnam War and helped fuel a decisive shift in public opinion.

“It hit people in the gut in a way that only a visual text can do,” said Michelle Nickerson, an associate professor of history at Loyola University Chicago who has studied the antiwar movement during the Vietnam era. “The photo translated the news of Tet in a way that you can’t quantify in terms of how many people were, at that moment, turned against the war.”

The execution happened on ese forces launched the coordinated attacks of the Tet offensive. Suddenly, insurgents were in dozens of cities, in almost every province of South Vietnam. They were in the streets of Saigon, the capital. They were even inside the heavily guarded compound of the United States Embassy.

It was a shocking sight for Americans, who had been assured by President Lyndon B. Johnson and his top general in Vietnam, William C. Westmoreland, that the enemy was on its last legs.

Meredith H. Lair, a Vietnam War expert at George Mason University, said the offensive “caused people to question whether they’d been fed lies by the administration, and to question whether the war was going as well as they’d been led to believe, and to question whether the war could be won if the enemy was supposed to be cowed and appeared so strong and invigorated.”

If the broader Tet offensive revealed chaos where the government was trying to project control, Adams’s photo made people question whether the United States was fighting for a just cause. Together, they undermined the argument for the war on two fronts, leading many Americans to conclude not only that it could not be won, but also that, perhaps, it shouldn’t be.

The photo “fed into a developing narrative in the wake of the Tet offensive that the Vietnam War was looking more and more like an unwinnable war,” said Robert J. McMahon, a historian at the Ohio State University. “And I think more people began to question whether we were, in fact, the good guys in the war or not.”

Nguyen Ngoc Loan,” he wrote in Time magazine

A police chief had fired a bullet, point-blank, into the head of a handcuffed man, in likely violation of the Geneva Conventions. And the official was not a Communist, but a member of South Vietnam’s government, the ally of the United States.

“It raised a different kind of question to Americans than whether or not the war was winnable,” said Christian G. Appy, a professor of history at the University of Massachusetts, Amherst. “It really introduced a set of moral questions that would increasingly shape debate about the Vietnam War: Is our presence in Vietnam legitimate or just, and are we conducting the war in a way that is moral?”

In the months after the Tet offensive, public opinion shifted more rapidly than at any other point in the war, Dr. McMahon said. Adams’s photo won a Pulitzer Prize, and Time magazine called it one of the 100 most influential ever taken.

“You can talk about ‘the execution photograph from the Vietnam War,’ and not just the generation who lived through it but multiple generations can call that image to mind,” said Susan D. Moeller, the author of “Shooting War: Photography and the American Experience of Combat,” and a professor of media and international affairs at the University of Maryland. “It was immediately understood to be an icon.”

Yet the decisions on how to display this photo and other graphic Vietnam war imagery were matters of debate in the newsroom of The Times. “I remember certain pictures,” the influential photo editor John G. Morris, who died at 100 last year, said, “which I was just determined to get on page one.” This was one, as was the 1972 picture of a naked 9-year-old, Phan Thi Kim Phuc, fleeing a napalm attack. That ran at the bottom of the page.

In South Vietnam, the execution image resonated in a different way. To Americans in 1968, it conveyed that North Vietnam and the Vietcong were far stronger than they had been led to believe. To South Vietnamese, it conveyed the opposite: Those forces “no longer had the kind of aura of omnipotence that they had had before,” said Mark Philip Bradley, a historian at the University of Chicago.

Then there was the fallout for the person for whom viewers had the least sympathy: General Loan, the executioner, who would eventually move to the United States. In 1978, the government tried unsuccessfully to rescind his green card. He died 20 years later in Virginia, where he had run a restaurant.

He noted that photographs, by nature, exclude context: in this case, that the prisoner had killed the family of one of General Loan’s deputies

“Two people died in that photograph: the recipient of the bullet and Gen. “The general killed the Viet Cong; I killed the general with my camera.”