The United States' involvement in Vietnam dates to the Geneva Accords, signed during President Eisenhower's administration:
According to the terms of the Geneva Accords, Vietnam would hold national elections in 1956 to reunify the country. The division at the seventeenth parallel, a temporary separation without cultural precedent, would vanish with the elections. The United States, however, had other ideas. Secretary of State John Foster Dulles did not support the Geneva Accords because he thought they granted too much power to the Communist Party of Vietnam.
Instead, Dulles and President Dwight D. Eisenhower supported the creation of a counter-revolutionary alternative south of the seventeenth parallel. The United States supported this effort at nation-building through a series of multilateral agreements that created the Southeast Asia Treaty Organization (SEATO).
Using SEATO for political cover, the Eisenhower administration helped create a new nation from dust in southern Vietnam. In 1955, with the help of massive amounts of American military, political, and economic aid, the Government of the Republic of Vietnam (GVN or South Vietnam) was born. The following year, Ngo Dinh Diem, a staunchly anti-Communist figure from the South, won a dubious election that made him president of the GVN. Almost immediately, Diem claimed that his newly created government was under attack from Communists in the north. Diem argued that the Democratic Republic of Vietnam (DRV or North Vietnam) wanted to take South Vietnam by force. In late 1957, with American military aid, Diem began to counterattack. He used the help of the American Central Intelligence Agency to identify those who sought to bring his government down and arrested thousands. Diem passed a repressive series of acts known as Law 10/59 that made it legal to hold someone in jail if s/he was a suspected Communist without bringing formal charges.
Also check out this letter from President Eisenhower to President Ngo Dinh Diem:
DEAR MR. PRESIDENT: I have been following with great interest the course of developments in Viet-Nam, particularly since the conclusion of the conference at Geneva. The implications of the agreement concerning Viet-Nam have caused grave concern regarding the future of a country temporarily divided by an artificial military grouping, weakened by a long and exhausting war and faced with enemies without and by their subversive collaborators within.
Your recent requests for aid to assist in the formidable project of the movement of several hundred thousand loyal Vietnamese citizens away from areas which are passing under a de facto rule and political ideology which they abhor, are being fulfilled. I am glad that the United States is able to assist in this humanitarian effort.
We have been exploring ways and means to permit our aid to Viet-Nam to be more effective and to make a greater contribution to the welfare and stability of the Government of Viet-Nam. I am, accordingly, instructing the American Ambassador to Viet-Nam to examine with you in your capacity as Chief of Government, bow an intelligent program of American aid given directly to your Government can serve to assist Viet-Nam in its present hour of trial, provided that your Government is prepared to give assurances as to the standards of performance it would be able to maintain in the event such aid were supplied.
The purpose of this offer is to assist the Government of Viet-Nam in developing and maintaining a strong, viable state, capable of resisting attempted subversion or aggression through military means. The Government of the United States expects that this aid will be met by performance on the part of the Government of Viet-Nam in undertaking needed reforms. It hopes that such aid, combined with your own continuing efforts, will contribute effectively toward an independent Viet-Nam endowed with a strong government. Such a government would, I hope, be so responsive to the nationalist aspirations of its people, so enlightened in purpose and effective in performance, that it will be respected both at home and abroad and discourage any who might wish to impose a foreign ideology on your free people.
Dwight D. Eisenhower