What prompted the us to enter into an all-out war in vietnam?

The U.S. had/has pledged to prevent the spread of communism and to support any nations fighting against a communist/tyrannical government

Britain and its empire stood alone against hitler and mussolini. in june 1941, hitler broke the non-aggression agreement with stalin and germany invaded the soviet union. in december, japan attacked the us and britain. the main lines of world war ii had formed.