Causes of Vietnam war

1. American anti-communism - the involvement of America into 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 of the south-east Asia will be under the control of the communists, in consequences this system will spread out to all countries around, just like in a domino game. (if one piece fall, it start the whole series of 'falling')
2. War between North and South - to this conflict were involved South (Republic of Vietnam) and North (Democratic Republic of Vietnam). South which seemed to be against communism were supported by USA and their allies South Korea, Thailand, Australia, New Zealand and Philippines and North were supported by communist countries as Soviet Union. As soon as South Vietnam started to lose America gave them big financial support which very quickly proved to be not enough. So they gave them military advisors but  as it 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 made an uprise??. Vietnam was then a French colony but France didn't even think about resign from the Indochina. As a result they fight bloody for several years. In the final stage of this war France was supported by USA even with american airman and military advisors. At the end Vietnam defeated France so they sign a peace treaty with communists from Viet Minh. After this Vietnam had been devided into two parts.