Forty years later, the images remain searing: Throngs of desperate South Vietnamese civilians trying to scale the walls of the U.S. Embassy in Saigon, hoping somehow to squeeze aboard one of the helicopters evacuating U.S. personnel and their associates in the face of an onslaught by North Vietnamese forces.
U.S. Ambassador Graham Martin, in fear of spreading panic, had delayed launching the evacuation until the last opportunity. As a result, potentially better plans for transport planes or cargo ships were scrapped - and the helicopters, with space for fewer refugees, were called in on April 29, 1975.
Fall of Saigon 1975 Vietnam War
40 years on, images of Saigon's fall remain indelible
FILE - In this 1966 file photo, U.S. Army helicopters providing support for U.S. ground troops fly into a staging area fifty miles northeast of Saigon, Vietnam. The war ended on April 30, 1975, with the fall of Saigon, now known as Ho Chi Minh City, to communist troops from the north. (AP Photo/Henri Huet, File)
A little girl in a pedicab, and her driver stare as they pass a demonstration of nine anti-war activists before the United States embassy in Saigon, Friday, Jan. 25, 1975. The activists, led by David Harris, left, of Menlo Park, Calif. former husband of folk singer Joan Baez, passed out leaflets demanding the end of U.S. intervention in South Vietnam. (AP Photo)
At old man and a child, refugees from the district town of Tri Tam, 40 miles northwest of Saigon on March 13, 1975, flee down the high as a South Vietnamese armored vehicle in the background heads toward the fighting the Saigon command said on Thursday, march that Tri Tam had fallen to the North Vietnamese after three days of hard fighting. (AP Photo)
Their household goods in a haphazard pile, a South Vietnamese refugee family takes time out for lunch at Da Nang in Vietnam, March 28, 1975, where they are waiting to be relocated after fleeing from the Northern provinces. (AP Photo/Phuoc)
Two stick-wielding plainclothes policemen fight South Vietnamese Buddhist nuns in Saigon, Sunday, Jan. 26, 1975 during anti government demonstration. Officer (left) warns comrade that nun (background) is about to hit him with sandal. Melee occurred following political convention at pagoda. (AP Photo/Nguyen Tu A)
Teeming frightened humanity crowds the decks of the merchant vassal Pioneer Contender as it docks at Cam Ranh Bay on the central coast of South Vietnam, Friday, March 29, 1975. Ship carried 5,600 South Vietnamese refugees and about 40 Americans out of Danang, a refugee crammed city under the gun. (AP Photo/Huynh Cong/Ut)
Father and mother, from front and back, help lift their youngster from ship to dock as the family and hundreds of other refugees arrive at Saigon dock, Friday, March 29, 1975. The arrival of the merchant vessel was one of the first at the capital since the mass exodus of South Vietnamese from the central highlands and Northern provinces. (AP Photo/Phuoc)
A South Vietnamese father carries his son and a bag of household possessions as he leaves his village near Trang Bom on Route 1 northwest of Saigon April 23, 1975. The area was becoming politically and militarily unstable as communist forces advanced, just days before the fall of Saigon. (AP Photo/KY Mhan)
Youthful demonstrators toss sticks and rocks at South Vietnamese riot police in Saigon, Thursday, April 4, 1975 during brief confrontation. Clash grew out of rallies sponsored by mainly Catholic anti-corruption movement. Tense situation in Saigon resulted in curfew change from 10 p.m. to 9 p.m. (AP Photo/Lo Vinh)
A Saigon student demonstrator hastily flees after a policeman grabbed his antigovernment banner, Thursday, April 9, 1975. About 30 students protested the draft and demanded the ouster of South Vietnamese President Nguyen Van Thieu. (AP Photo/Dang Van Phuoc )
Le Thi Yuan, a Saigon housewife, weeps moments after told that the Northern City of Da Nang had fallen to communist forces in Saigon on March 30, 1975. Her husband, a soldier, was in Da Nang when the port city fell. With her is their six-year-old son. (AP Photo/Zeitlin)
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, Tuesday, April 29, 1975. The helicopter had carried Vietnamese fleeing Saigon as North Vietnamese forces closed in on the capital. (AP Photo)
FILE - In this April 30, 1975 file photo, a North Vietnamese tank rolls through the gate of the Presidential Palace in Saigon, signifying the fall of South Vietnam. Vietnam will celebrate its 34th anniversary of the end of the Vietnam War on April 30, 2009. (AP Photo/File)
A U.S. Marine helicopter takes off from helipad on top of the American Embassy in Saigon, Vietnam, April 30, 1975. (AP Photo/Phu)
South Vietnamese refugees disembark at Cam Ranh Bay in Vietnam, Saturday, March 30, 1975 on South Vietnam?s central coast following a voyage from Da Nang, the city which has since fallen to communist forces. Westerner at right is a member of the ship?s crew. (AP Photo/Nick Ut)
FILE -- Mobs of Vietnamese scale the walls of the U.S. Embassy in Saigon in an effort to get to a helicopter pick up zone as the capital of South Vietnam was falling to North Vietnamese forces May 1, 1975. Restoring the American diplomatic presence in Vietnam, Secretary of State Warren Christopher will preside over the opening of the U.S. Embassy, this time in Hanoi, the capital of the nation unified in 1975. (AP Photo/Neal Ulevich)
Mobs of South Vietnamese civilians scale the 14-foot wall of the U.S. Embassy in Saigon, April 29, 1975, trying to reach evacuation helicopters as the last Americans departed from Vietnam. (AP Photo)
A South Vietnamese woman weeps as she and others flee for the safety of the capital area of Saigon from a rural district near the city on April 28, 1975. The rapid advance of communist troops, however, leaves the security of Saigon in serious doubt. (AP Photo)
Unidentified members of a television news crew run for cover in Saigon on Monday, April 28, 1975 during fighting between South Vietnamese Government troops and Communist forces on the northern outskirts of Saigon. (AP Photo/Thauh Nuy)
Refugees fleeing the advance of communist forces create a massive traffic jam on the northern outskirts of Saigon April 28, 1975 as South Vietnamese troops searched vehicles for communist infiltrators. (AP Photo/H. Hung)
South Vietnamese civilians, some of them cover their faces from being photographed, line up at the United States Consulate in Saigon, South Vietnam, on April 24, 1975, to seek evacuation before the Communist North Vietnamese troops seize power in the capital. (AP Photo)
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By late afternoon, perhaps 10,000 desperate Vietnamese had converged on the embassy, many of them fearful of retaliation by the North Vietnamese for their cooperation with the U.S. during the long war.
There were other designated pickup points in Saigon besides the embassy, but helicopters did not reach them all, leaving behind some of the most vulnerable Vietnamese, many of them CIA workers.
Some planes that did try to leave Saigon's airport were shot down.