by MATTHEW GAULT

Graham Martin, the U.S. ambassador to South Vietnam, loved the tamarind tree that grew in the parking lot outside America’s embassy in Saigon. “As steadfast as the American commitment in Vietnam,” the diplomat often described it.

On the morning of April 29, 1975, the CIA station chief told Martin the tree would have to come down. Personnel needed to clear out all the shrubbery in the lot to make room for helicopters.

The building’s roof was the only viable helipad, which wasn’t enough if the ambassador wanted to evacuate thousands of Americans and Vietnamese.

Marines cut down the tree, creating a makeshift landing zone. The symbolic act was the final blow to Martin—who clung to a dream of an independent South Vietnam far longer than was rational.

A little after noon the same day, Marine Capt. Gerald Berry landed a helicopter on the embassy’s roof. He had orders to get Martin out, but the ambassador refused. Thousands of South Vietnamese milled on the grounds of the embassy. More pressed against the fences, looking for a way in.

“I feel a very heavy moral obligation to evacuate as many deserving Vietnamese as possible,” he wrote in a memo to then-secretary of state Henry Kissinger.

Here’s just one of the many vivid, heartbreaking and morally complicated scenes in Rory Kennedy’s documentary Last Days in Vietnam. The film is up for an Oscar for best documentary, and it’s easy to see why.

The film details the final moments of the Vietnam War and focuses on America’s embassy in Saigon. Kennedy and her team tell the story using film footage, letters, voice recordings and interviews with soldiers, politicians and survivors.

The film’s greatest strength is that all the narration comes from people who were there. The filmmakers let the witnesses tell the story.

Kennedy uses computer generated maps to illustrate the events, but never breaks in with her own voice. The film does editorialize, but it does so through the voice of those affected—a choice that makes the film both honest and powerful.

The survivors tell dozens of fascinating stories in Last Days in Vietnam, and the most poignant is Martin’s. The embattled ambassador waited until the last possible hour to begin the evacuation.

Martin lost a foster son to combat in Vietnam, and some of the survivors speculate that this clouded his judgement.

The ambassador wanted to believe America would stay behind forever, to make the sacrifices of so many families mean something.

The North Vietnamese Army pushed south throughout April 1975. The U.S.-backed southern army collapsed, yet Martin never believed the communists would take Saigon.

When North Vietnam bombed the city’s main airport—closing off the principle means of escape—Martin refused to believe it until he saw it with his own eyes.