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.