The Vietnam war had multiple long term and short term causes, many of which were worldwide events as opposed to localised incidents.
Perhaps the most prevalent cause was the US's fear of communism. Indeed, the beginning of the cold war in the late 1940’s started a chain of events that would change the United States forever. In the US the cold war was seen by public officials as a war on communism, and this inspired an America wide fear of communist takeover. The US and USSR could not risk a physical war, due to the nuclear weapons both sides were known to possess, and so the war was thought with words and through other 'client' countries. Vietnam was one of these client countries, and was a large battleground for the Cold War. The US were fighting on the ground, which meant the USSR could not; this led to the USSR supplying neighbouring China with weapons, which were in turn supplied to the North Vietnamese fighters.
The French involvement in the Vietnam War is a factor which cannot be overlooked. The French had colonised Vietnam, and were exploiting the countries raw materials and economic markets. However, the Vietnamese generally wanted to be free from French colonial rule, a fight which had been going on for several decades. As the French forces were weakened, they called on their American allies for help.
Ho Chi Minh returned to Vietnam in 1941, and embraced communist ideals. He established the Viet Minh and the National Liberation Committee, and fought for the independence of Vietnam. America, fearing the spread of communism, refused to acknowledge this independence. The cause for this is often linked to the domino effect. It was believed by Americans that if one country in Indochina were to turn communist, the rest would follow. They feared this would cause an Asian or possibly World wide spread of communist takeover.
The attempted declaration of Vietnamese Independence is what would 'fire the gun' for the start of the Vietnam War, determined to keep their Vietnamese colonies, the French seized Haiphong and Langson in November of 1946. This is generally accepted as the start of the war.
The American involvement in the Vietnam War grew rapidly in the beginning years. In 1949 Communist rule was established in China, and it was feared that such a nearby communist influence would only increase support for a communist rule in Vietnam. The western world supported Bao Dai, while Eastern Europe and the Russians supported Ho Chi Minh as the leader.
In July 1954 it was decided to split Vietnam into North and South, a split which would only escalate the war. North Vietnam quickly sent troops to infiltrate the south and spread the ideas of communism, whilst South Vietnam was ruled by a strongly anti-communist leader, Ngo Dinh Diem, who had the support of the Americans. The Viet Cong developed in the north, using the heavily jungle covered Ho Chi Minh trail to attack American and South Vietnamese soldiers. The guerilla tactics used by the Viet Cong and eventually the NVA meant the war was different to anything the American soldiers had previously experienced. The Viet Cong were trained by their commander Giap who learned from the tactics used by the Chinese communists in their fight against the Nationalist Chinese forces. He expected his troops to fight and to help those in the south. He introduced a "hearts and minds" policy long before the Americans got militarily involved in Vietnam.
These events and new fighting styles caused the Vietnam War to become the only war ever lost by the USA.