The French wanted to capture Vietnam as a whole- the French were defeated at Dien Bien Phu and caused Vietnam to split at the 17th Parellel
The North and South split because the South didn't like the way the North was ruling their government; with communism.
The United States helped France try and take over Vietnam because they thought it would help stop Vietnam from falling to communism
The United States joined in with South Vietnam because they were trying to stop the spread of communism
Another event that occurred at this time was the formation of the Southeast Asia Treaty Organization (SEATO) in 1955. The charter nations of SEATO included Australia, France, New Zealand, Pakistan, the Philippines, Thailand, the United Kingdom, and the United States. SEATO was a “collective defense arrangement calling for mutual help and consultation to resist overt Communist aggression or other acts threatening international security.” SEATO would later include South Vietnam, Laos, and Cambodia among its members. SEATO created problems, especially when South Vietnam was included, as the Geneva Conference stipulated that neither North nor South Vietnam could join into alliances with foreign powers.