There were two main causes of the Vietnam War. One cause had to do mainly with the Vietnamese and one cause had mainly to do with the Americans.
On the Vietnamese side, the main cause of the war was the desire to have a national government that was truly independent. Vietnam had long seen itself as a country that deserved to be independent. Even so, it had been colonized for a very long time, first by China, then by France, and then for a short while during WWII, by Japan. With this history, Vietnam was eager to have true independence. The group that had the most credibility as a nationalist force was the Vietminh, led by Ho Chi Minh. Unlike the regime in South Vietnam, this regime did not appear to the Vietnamese to be beholden to foreign powers. To make a united Vietnam that was not (in his mind) under foreign domination, Ho encouraged a conflict against the South Vietnamese government.
On the American side, the main cause of the war was the fear of communist expansion. Ho Chi Minh was a communist and American leaders believed that he would be controlled by the Soviet Union. They believed that a communist Vietnam would lead to a “domino effect” in which more and more countries in East Asia would become communist, leaving the communists to dominate that area of the world. To prevent this, the US went to war.
Since the other educator has already outlined the main causes for the Vietnam War, I will provide further details to support his arguments.
In relation to the American side, it is true that the United States feared communist expansion in South East Asia. Basically, the Vietnam War represented a power struggle between Russia and the United States for global dominion. While the United States favored capitalist open market economies, Russia's Communist government supported closed-state economies. China, which had just fallen to Communist control on October 1, 1949, became Russia's natural ally.
Long before American soldiers set foot on Vietnamese soil, the Russians and Chinese were already working to prop up Ho Chih Minh's insurgency in North Vietnam. For his part, Ho Chih Minh yearned for a united Vietnam that was governed by Communist ideology.
The Russians fought the United States indirectly during the Vietnam War by funneling financial support through China to Vietnam. With Russia's financial backing, the Chinese were able to provide logistics and weapons support to North Vietnam. After WWII, the USA and the USSR emerged as the most powerful nations on the world stage, and both nations became engaged in a Cold War between 1945-1991. The USSR had no intentions of openly revealing its support for Communist China or its plans for global dominance. Instead, it quietly supported Ho Chih Minh's communist revolution through the Chinese. During the Vietnam War, Russia sent no Russian soldiers to the war front; instead, it equipped the North Vietnamese with SAMs (Surface-to- Air-Missiles) capable of bringing down American planes from altitudes of more than 60,000 feet. Additionally, North Vietnamese MiG fighter pilots were trained by Russian veterans of WWII.
The United States suspected Russian involvement in Vietnam's civil war almost immediately, and this proved to be the main factor for its own foray into the Vietnam War. The Americans feared that Communist dominance in South East Asia would strengthen Russia's global presence. So, various American presidents took steps to stop what they viewed as Russian aggression in the region.
President Eisenhower sent military advisers to train the South Vietnamese army; he also directed the CIA to conduct psychological warfare on the North Vietnamese. Before his death, President Kennedy sent 400 Special Operations Green Berets to train the South Vietnamese in counter-insurgency warfare. Kennedy's successor, Lyndon Johnson, formally authorized American entry into the war. So, the war was essentially fought over two differing ideologies: communism and capitalism.
Source: LIFE magazine, Sept 9, 1966