In 1960?
President Dwight D. Eisenhower wrote to the new Prime Minister of the Bao Dai government, Ngo Dinh Diem, promising United States support to ensure a noncommunist Vietnam. Following through on that commitment, direct United States aid to South Vietnam began in January 1955, and American advisors began arriving in February to train the South Vietnamese army.
I think it was 1955; after Dien Bien Phu in 1954.