Causes of US involvement in Vietnam war:1) Major cause to prevent the spread of communism .2) US wanted to establish democracy in South Vietnam .3) North Vietnam led by communist party and they tried to influence the southern Vietnam . Americans believed that, if South Vietnam fell, Laos, Cambodia and Thailand – and then Burma and India – would follow.So they entered into war.4) It was obvious the South Vietnamese could not resist communist infiltration by the North Vietnam without help and also US was supporter of democracy so it took part in war to counter the communism.