Why did the veitnam war start?
because America wanted to stop the Communism in the country
The war started because the french were unwilling to grant Vietnam independence which was what the U.S. were preaching at the end of world war II, ironically they ending up supporting the french for reasons outlined below.
The U.S. firstly were reluctant to enter Vietnam. They HAD to support the French as they were vital in the rebuilding of Europe after WWII. Beyond this just a few things to keep in mind is that Ho Chi Minh worked with the U.S. during and just after WWII and he only turned to the communists when the U.S. decided to support the french. There is significant evidence that if the French did not violently enforce their colonial domination over Vietnam and gave them independance (just as Roosevelt proposed at the end of WWII) Ho chi minh could of been the next Tito. In other words on the Americans side more or less.
Therefore, in many ways the French caused the US to become involved in the Vietnam war.