PLEASE HELP WITH TERM PAPER ON VIETNAM

HI, im writing a term paper for my history class on the US puppet governments and the cause of the war. the basic thesis that ive come up with so far in my research is something like this: "the cause for US declaration of war came from the need to protect its investment in Vietnam, the puppet government of Ngo Dinh Diem" please tell me if this is totally off and what would be a more accurate topic/thesis including the US puppet governemnts and if you tihnk its accurate, what kind of support i should use and what liturature you would recommend. thanks alot