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

What were the causes of the Vietnam War?

The roots of the Vietnam War go back to the end of World War II as Vietnamese nationalists under Ho Chi Minh waged a successful anti-colonial war against the French. After the French were kicked out of the country following the siege at Dien Ben Phu, the United Nations created North and South Vietnam, with the North sponsored by the Soviet Union and the South looking to the United States for support. The plan was to have the two sides unified after a special election. Ho Chi Minh, leader of the communist North was very popular, while the leader of South Vietnam, Diem, maintained too many traits of French colonialism for some people. The United States backed him because he was anticommunist. When Diem blocked the election because he said that the North would cheat, there were protests all over Vietnam, and Ho Chi Minh stepped up his campaign to link the country by force. Diem was later assassinated, and a string of corrupt and inefficient leaders followed. The United States sent advisers and money to South Vietnam as early as the Eisenhower administration. This only increased under the leadership of John F. Kennedy, as he relied on sending more advisers. The Vietnam War truly escalated for the United States after the USS Maddox was attacked in the Gulf of Tonkin by North Vietnamese forces. Congress soon signed the Gulf of Tonkin Resolution, giving Lyndon Johnson a blank check to commit thousands more ground troops to fight in Vietnam.

What were the major causes of the Vietnam War?

From the Vietnamese side, the main cause of the Vietnam War was the desire of the Vietnamese to have a country that was free from excessive foreign influence.  The North Vietnamese and the Vietcong in the South both felt that the government of South Vietnam was essentially a colonial government.  The Vietminh had fought against French colonialism and they did not want to simply trade it for what they saw as American colonialism.  Therefore, they continued to fight the Americans after the French were gone.

From the American point of view, the main cause of the Vietnam War was the desire to prevent the spread of communism.  Ho Chi Minh, the leader of the North, was a communist.  The US felt that this meant that any country he governed would essentially be run by the Soviet Union.  The US felt that allowing communism to spread in this way would give the Soviets too much power.  The US feared a “domino effect” that would lead to the fall of other countries in East Asia.  This would help the communists to gain more power in the world.  The desire to prevent this led the United States to fight the Vietcong and the North Vietnamese.