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The Associated Press
In this Tuesday, April 29, 1975 file photo, American citizens arrive aboard the command and control ship USS Blue Ridge after being evacuated out of Saigon, South Vietnam, by U.S. Marine and Air Force helicopters operating from Navy ships.

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.

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.