Operation FREQUENT WIND
Operation FREQUENT WIND was the code name for the helicopter evacuation by the U.S., of its own nationals, and some South Vietnamese, from the Saigon area, as the Republic of Vietnam was overrun by conventional invasion by the People's Army of Viet Nam (i.e., the North Vietnamese military). The final evacuation came on April 30, 1975; other, less dramatic evacuations had been taking place as the PAVN advanced. Previously, on April 12, U.S. and allied personnel were evacuated from Phnom Penh, Cambodia, in Operation EAGLE PULL.
Many South Vietnamese escaped by boat, but many others did not. Before Tan Son Nhut airfield was overrun, other Vietnamese escaped from that main Saigon airfield, as others had been escaping from other airbases before the advancing PAVN.  Many Vietnamese were imprisoned or executed by the PAVN.
A great amount of records about South Vietnamese who had worked with the U.S., incriminating in North Vietnamese eyes, were not destroyed as they reasonably should have been. Further, South Vietnamese employees of the Central Intelligence Agency, who would be at high risk, were not evacuated. 
The 9th Marine Amphibious Brigade, under the tactical commmand of Alfred M. Gray, Jr., would enter Saigon to evacuate the last Americans from the American Embassy to ships of the Seventh Fleet. Ambassador Graham Martin was among the last civilians to leave. 
- Goldstein, Donald M.; Katherine V. Dillon & J. Michael Wenger (1999), The Vietnam War: The Story and Photographs, Brassey's, pp. 153-160
- Thomas, Evan (May 1, 2000), "The Last Days Of Saigon; Special Report: The Communist Takeover Of South Vietnam's Capital Was A Low Moment In The American Century.", Newsweek
- Shulimson, Jack, The Marine War: III MAF in Vietnam, 1965-1971, 1996 Vietnam Symposium: "After the Cold War: Reassessing Vietnam" 18-20 April 1996, Vietnam Center and Archive at Texas Tech University