In this April 29, 1975, photo, a helicopter lifts off from the U.S. Embassy in Saigon, Vietnam, during last-minute evacuation of authorized personnel and civilians. (AP Photo/File)

In the waning moments of the U.S. war in Vietnam, U.S. Ambassador Graham Martin called for an evacuation of the U.S. Embassy in Saigon. North Vietnamese forces were rolling south toward the city, and thousands of Americans and South Vietnamese needed to be evacuated.

What followed was Operation Frequent Wind. The U.S. military pulled more than 7,000 people from Saigon, now known as Ho Chi Minh City, in helicopters on April 29 and 30, 1975. Images of helicopters lifting off from the roof of the embassy remain burned in the memory of many Americans, a symbol of the pain and frustration of the Vietnam War.

To mark the anniversary, here are images from that operation. The United States used so many helicopters to evacuate people, some were pushed overboard from Navy ships off the coast of Vietnam to allow more to land. Thousands of South Vietnamese who had helped the United States were left behind.

In this April 29, 1975, photo, South Vietnamese civilians scale the 14-foot wall of the U.S. Embassy in Saigon, trying to reach evacuation helicopters as the last Americans depart from Vietnam. (AP Photo/File)

U.S. Navy personnel aboard the USS Blue Ridge push a helicopter into the sea off the coast of Vietnam in order to make room for more evacuation flights from Saigon on April 29, 1975. The helicopter had carried Vietnamese people fleeing Saigon as North Vietnamese forces closed in on the capital. (AP Photo/File)

American citizens arrive aboard the command and control ship USS Blue Ridge on April 29, 1975, after being evacuated out of Saigon, South Vietnam, by U.S. Marine and Air Force helicopters operating from Navy ships. (U.S. Navy via AP, File)

A North Vietnamese tank rolls through the gates of the Presidential Palace in Saigon, on April 30, 1975, signifying the fall of South Vietnam. (AP Photo/File)

This April 30, 1975, photo provided by former U.S. Marine Master Gunnery Sgt. Juan Valdez of Oceanside, Calif., shows him, rear center, sitting on the last helicopter leaving the U.S. Embassy in Saigon, Vietnam. (AP Photo/Courtesy of Juan Valdez via AP)