Starting in 1862 the French took over Vietnam and colonized it divided into three countries as shown in the map picture to the left; Tonkin, Annam and Cochin-China.
During World War II France was occupied by Nazi Germany, it lost its anchors in Vietnam and Japan took control of the country as of 1940. As a result Ho Chi Minh formed a militant nationalist organization; the League for the Independence of Vietnam commonly referred to as the guerrilla group "Viet Minh". Japan surrendered at the end of World War II in 1945, Ho Chi Minh’s forces claimed the capital of Hanoi and September 1945 declared Vietnam to be an independent country; the Democratic Republic of Vietnam. France however ignored the declaration and re-entered Vietnam which moved the Viet Minh forces Northern. Ho asked Dwight David Eisenhower the 34th President of the United States (who served two terms from 1953-1961) for help against the French, he revoked getting involved because at the time we were in the Cold War with the Soviet Union/USSR.
The Cold War began 1947 after WWII and was referred to as the 'cold war' because there was no major armed combat, but it didn't end until 1991. There was a major increase in mass destruction weapons such as the nuclear bomb, the threat of such happening resulted in many fallout precautions like nuclear school drills and bomb shelters. It was between the two super powers of the Soviet Union (this was in the time of the rise of Hitler) also known as the USSR and the United States. This encouraged prideful race competitions between various advancements in science, the space race etc.
The Viet Minh and French forces attacked each other from 1946-1954 until the Viet Minh overtook the French in the Battle of Dien Bien Phu in 1954; this marked the year France pulled out of Vietnam. Then came the Geneva Conference that year which declared a ceasefire and for Vietnam to be split into Northern and Southern sections with the intent of a general election to be held by July 1956 when one would be voted into office as president and re-unify the country. The North fell under communist Ho's rule and in fear of communism spreading across all of Southeast Asia known as the domino theory (that one communist country would lead to a chain reaction of political change) the US supported the anti-French colonialist/anti-Communist politician Ngo Dinh Diem's rise to power of the South Vietnamese government in 1955. It's often thought that the elections were fixed as Diem had supporters strong arming others into a forced vote and the turn out in the capital Siagon was more than 600,000 out of the 450,000 on the electoral roll at the time. After he was deemed as president he declared the Republic of Vietnam and postponed the election originally set for 1956 because of extensive communism influence (it was clear Ho would have won). Though the US viewed him as politically correct Diem upset the people of South Vietnam and had a minute following because he ruled with strict oppressive and corrupt policies. He ignored United States advocations in claims that he refused to be a "US puppet", but despite his deficiencies as a leader we continued to supported him because of our common political stance. Ho was widely popular with the masses known for his charismatic wisdom, he was quoted in a speech saying “If people in an independent country do not enjoy happiness and freedom then independence has no meaning.”
In 1961 president Eisenhower's two terms were up and he was followed by John F. Kennedy who supported his stance to support by any means the containment of communism. Kennedy decided to financially support a militia growth from 150,000 to 170,000 and train the soldiers by sending an additional 1000 US military advisers to South Vietnam, this broke the 1954 Geneva Agreement and was not publicized. In an attempt to bypass communist insurgency Diem forced the relocation of common peasantry; this isolated them from influence and was hoped to strengthen the peoples faith in the South Vietnamese government providing safety and physical isolation. This was known as the Strategic Hamlet Program and it had the opposite effect, in fact many began supporting the communist efforts over Diem's and fought against his rule. One of the most well renowned and publicized protests against Diem was related to religious oppression of specifically Buddhism which was a supported practice by the majority between a rough estimate of 70-90 percent of the population. He refused to revoke anti-Buddhist laws previously passed by the French and one devout monk in particular, Thích Quảng Đức fought the religious persecution by a self-immolation suicide on a busy road in Siagon, he was willing to die to make a statement and he succeeded. When Kennedy recognized that he would no longer be capable of unifying South Vietnam he backed his assassination which occurred November 1963 and three weeks later was assassinated himself, some believe this to be connected.
After Kennedys death his successor Lyndon B. Johnson respected his take on involvement but intended to keep such to a minimum. However, after the Gulf Tonkin Incident of 1964 allegedly involving North Vietnamese parties there was the Gulf of Tonkin Resolution which gave Johnson the legal authority and served as a social justification to the public to deploy American troops and initiate in armed combat. There was a lot of propaganda and twisting of the situation in use to convince the general public that our involvement was necessary as a moral service to the innocent South Vietnamese citizens.