About this image. From BBC
"There's something in the nature of a still image that deeply affects the viewer and stays with them," says Ben Wright, associate director for communications at the Dolph Briscoe Center for American History.
The centre, based at the University of Texas at Austin, houses Adams's archive of photos, documents and correspondence.
"The film footage of the shooting, while ghastly, doesn't evoke the same feelings of urgency and stark tragedy."
But the photo did not - could not - fully explain the circumstances on the streets of Saigon on 1 February 1968, two days after the forces of the People's Army of Vietnam and the Viet Cong launched the Tet Offensive. Dozens of South Vietnamese cities were caught by surprise.
Heavy street fighting had pitched Saigon into chaos when South Vietnamese military caught a suspected Viet Cong squad leader, Nguyen Van Lem, at the site of a mass grave of more than 30 civilians.
Adams began taking photos as Lem was frogmarched through the streets to Loan's jeep.
Loan stood beside Lem before pointing his pistol at the prisoner's head.
"I thought he was going to threaten or terrorise the guy," Adams recalled afterwards, "so I just naturally raised my camera and took the picture."
Lem was believed to have murdered the wife and six children of one of Loan's colleagues. The general fired his pistol.
"If you hesitate, if you didn't do your duty, the men won't follow you," the general said about the suddenness of his actions.