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In late 1954, Gen. J. Lawton Collins was made ambassador to "Free Vietnam" (the term South Vietnam came into use in 1955), effectively elevating the country to sovereign status. Collins' instructions were to support the leader Ngo Dinh Diem in subverting communism, by helping him to build an army and wage a military campaign. In February 1955, Eisenhower dispatched the first American soldiers to Vietnam as military advisors to Diem's army. After Diem announced the formation of the Republic of Vietnam (RVN, commonly known as South Vietnam) in October, Eisenhower immediately recognized the new state and offered military, economic, and technical assistance.

What was South Vietnam called in 1955?
Free Vietnam