March 8, 1965 -- The first US. combat troops arrived in Vietnam, as 3500 Marines landed at China Beach to defend the American air base at Da Nang. They joined 23,000 American military advisers already in Vietnam.
April 30, 1975 -- The last Marines of the force guarding the U.S. embassy depart Vietnam.
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May 1961 -- President Kennedy sends 400 American Green Beret 'Special Advisors' to South Vietnam to train South Vietnamese soldiers. This was the first deployment of a significant number of advisors to Vietnam, even though under Eisenhower there were some U.S. military advisors in Vietnam; two of whom were killed in 1959, the first American casualities in the conflict.
February 1962 -- The esablishment of the the United States Military Assistance Command, Vietnam under Army General Paul D. Harkins. By the end of the year, more than 12,000 U.S. military personnel (technicians, advisors, pilots, and supply and administrative personnel) were in Vietnam. Among this number were 18 Marine advisors to the South Vietnamese Marine Corps and a Marine helicopter task group, code named SHUFLY, consisting of a helicopter squadron and support element.
And a note of clarification to the first post, The Marine presence in Vietnam was withdrawn in 1971, but returned briefly in 1975 to evacuate Saigon and attempt to rescue the crew of the Mayaguez (a U.S. merchant ship seized by the Communist regime controlling Cambodia).
I did find one reference that states that as early as 1954 there was a Marine presence in Vietnam, and that for the following eight years Marines served in some advisory capacity there. This would indicate that there were some Marines in Vietnam in 1960. You can click here to view this article.
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