Causes of Vietnam war

1. American anti-communism - America's involvement in Vietnam started because it was afraid of communism. As soon as the war broke out between south and north they started to be afraid of Vietnam becoming a communist country because this political system would start to dominate in most Asian countries. (according to domino theory). The domino theory suggests that if one country in south-east Asia is under the control of the communists, then consequently this system will spread out to all countries around, just like in a domino game. (if one piece falls, it starts the whole series 'falling')
2. War between North and South - this conflict involved the South (Republic of Vietnam) and the North (Democratic Republic of Vietnam). The South, which seemed to be against communism, was supported by the USA and their allies South Korea, Thailand, Australia, New Zealand and the Philippines. The North was supported by communist countries such as the Soviet Union. As soon as South Vietnam started to lose America gave them big financial support which very quickly proved to be not enough.  They then gave them military advisers, but as this still didn't help they then started to send American soldiers. 
3. French return in 1945 and Vietnamese resistance - 2 September 1945 the Democratic Republic of Vietnam was created when the Viet Minh overthrew the government. Vietnam was then a French colony but France didn't even think about resigning from the Indochina. As a result they was bloody fighting for several years. In the final stage of this war France had USA support including American airman and military advisers. At the end Vietnam defeated France so they signed a peace treaty with communists from the Viet Minh. After this Vietnam was divided into two parts.