After learning of Ho Chi Minh friendship with communist China, America lost its sympathetic view that Vietnam was just struggling for its independence from France and began to back the French with $500 million a year. The US also helped setup an anti communist government in the south of the country. However, at this point America played a rather insignificant part in the war in Vietnam as they believed a rich European country would easily overcome Vietnam and set up a democratic, anti communist government.
They were wrong. In 1954 the French suffered heavy casualties at Dien Bien Phu, resulting in France's withdrawal. A conference held in Geneva meant that the country was temporarily divided into North and South Vietnam for a period of two years after which there were to be national elections and the reunion of the country. By this point the United States was significantly worried and they prevented the elections from taking place. They believed the communist party would easily gain a majority in the elections which could result in the feared domino effect taking place in South East Asia. Between 1950 - 54 American citizens were heavily influenced by the actions of Joseph McCarthy who attacked many key Government figures for being communist. Therefore, if the United States allowed Vietnam to hold elections, which brought in a communist Government then high profile figures like, President Eisenhower faced the embarrassment of being accused of not standing up to communism. This would mean that even though untrue, they would lose the backing of the American voters who were strong believers in McCarthy and anti-communism so that Eisenhower's party would not be re-elected in the next US elections.
The Viet Minh dominance and victory over a powerful West European state could give communist parties around the world the confidence and determination to gain control of their countries by force.
As a result of these factors in 1955, America helped the Catholic Ngo Dinh Diem set up the republic of South Vietnam. The US realised that Diem treated his people, especially the peasants most of whom were worshipers of Buddhism, cruelly and with contempt, but still allowed him to be leader. This shows how desperate the United States was for the country to be anti-Communist as they were prepared to allow a vicious dictator to rule. During 1955 America sent a team of advisors to mount a propaganda campaign to highlight false accusations that Ho Chi Minh was killing thousands of political opponents in the north of the country. When this failed America ploughed $1.6 billion to support Diem's regime. In the 1950's however Diem was widely hated by his people because of his cruel actions therefore in 1963 Ngo Dinh Diem was overthrown by his general with suspected but never proven support from the United States.
In 1960 John F. Kennedy was elected as President of the United States. We see Kennedy's willingness to continue America's efforts against communism in Vietnam when he said America would be willing to "pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and success of liberty." After this, the United States involvement escalated during 1961 when Kennedy gave South Vietnam the money to increase their army to 170,000. He also sent American advisors into the country to help train the army. This was because North Vietnam had set up the National Front for the Liberation of South Vietnam (Vietcong) in 1960 and the United States were becoming increasingly concerned that the South were unprepared and a weak force against the Vietcong's guerrilla tactics. The Domino theory was still widely believed in and ordinary Americans were still strong anti-Communists.
The US was scared of the influence of the Vietcong on the peasants of south Vietnam therefore the Strategic Hamlet program was introduced in 1962. This operation was intended to stop any co-operation between the peasants and the Vietcong by moving peasants into new villages which had a stockade built around it and were patrolled by armed guards. This failed dismally as peasants were forced to build fences and moats around their villages and now had to travel longer distances to reach their rice fields resulting in more peasants joining the Vietcong. This failure caused Kennedy to bring the total of advisors in Vietnam to 12,000 and give the country 300 helicopters. He did not want to go to full-scale war with Vietnam because the American public was against it at the time so soon after the Korean War. They felt as well as the rest of the world that Kennedy did not have the justification to go to war with Vietnam. In 1962 The United States was on the brink of nuclear war with the USSR as a result of the Cuban missile crisis and therefore his attention was very much focused on that. He would not have wanted to go to war with North Vietnam, as it was possible that a war was coming against the USSR where he would need his best troops.
After Kennedy's assassination in 1963, Lyndon Johnson became President and he was more prepared to commit to full scale war which is another factor in increasing Americas involvement in Vietnam. Johnson said, "he wasn't going to be the President that saw Vietnam go the way China did" (to communism).
When North Vietnamese patrol boats fired upon an American warship - The Maddox - in the Gulf of Tonkin, the US Congress gave President Johnson the power to take all necessary measures to prevent further aggression. Now that he had support of the American public as well, Johnson faced no initial opposition, which was a major factor in allowing the US to start sending combat troops to Vietnam and escalating their involvement in the countries affairs. On March 1965 3500 US combat troops came ashore at Da Nang thus starting the war with Vietnam.
In order to defeat the Vietcong, the United States launched Operation Rolling Thunder in February 1965 where extensive bombing raids were made on military and industrial targets in North Vietnam. There were more bombs dropped on Vietnam than in the whole of the Second World War. This tactic employed by the United States shows they were fully committed to winning the war.
To conclude I have highlighted several factors which increased the United States involvement in Vietnam. However, I believe the key factor was the Americans belief of the domino effect where a conspiracy was taking place in China and USSR to get communist governments to power. The US believed that if Vietnam were to fall to communism then it would be the first in a long line of countries such as Laos and Cambodia to fall as well like a line of dominoes.
