On May 7th 1954, Viet Minh’s forces won the Battle of Dien Bien Phu and ended French involvement in Indochina. This victory led to the Geneva Conference, where French and Viet Minh negotiated a ceasefire agreement.
Under the terms of Geneva Accords, Vietnam was temporarily divided at the 17th parallel where civilians were able to move freely between two states for a 300-days period. Elections were to be held two years later, in 1956, to establish an unified government.
But this Accords did not please the United States government. First, they feared that the elections could not be fair and free under Communist influence. Second, if Communist had won in Vietnam, Communism would spread to South East Asia and become a greater threat. In 1955, after Ngo Dinh Diem ousted Bao Dai and became the president of the Republic of Vietnam (ROV), the United States began to support ROV by sent the Military Assistance Advisory Group (MAAG) to the South Vietnam.
At the beginning in 1955, Diem started many anti-communist campaigns in all over South Vietnam. A lot of anti-government activists and communists were arrested. By the end of 1958, around 40,000 political prisoners has been jailed. In return, the National Liberation Front (NLF) (better known as Viet Cong) was established on consisting all anti-government activists including communists and non-communists.
In spite of a lot of efforts against communist and anti-government activists, Diem did not show that he was able to defeat the communists and some policymakers in Washington even believed that he might contact with Ho Chi Minh. In the middle of 1963, Washington began discussing about changing the regime. On 2 November 1963, Diem was assassinated and overthrown. Taking the advantage, North Vietnam increased their support for the guerrillas tactics.
On 24 November 1963, Lyndon B. Johnson, the then President of United States after Kennedy’s assassination, said “strength” and “determination” must be used in the battle against communism to express his solid viewpoint toward the war in Vietnam. One year later, the Gulf of Tonkin incident (the USS Maddox incident) occurred, involving three North Vietnamese Navy boats. This incident urged the Congress to approve the Gulf of Tonkin Resolution on 7 August 1964, giving the President more power to direct military operations.
At the beginning in 1965, Washington began to launch some bomb campaign to force North Vietnam to stop their support for NLF. On 8 March 1965, 3,500 US Marines were sent to South Vietnam marking the start of American ground war. US troops then increased rapidly to nearly 200,000 in December. At the time, Washington really made a serious move into Vietnam War.
Source and read more: thevietnamwar.info