Reason four - The weak South Vietnamese Government
As a result, much of South Vietnam's population was rebelling against him. The peasants wanted communism and supported the Vietminh and the NLF. In 1963, President John F Kennedy sent 16,000 military 'advisers' to help the South Vietnamese army.
China had become communist in 1949 and communists were in control of North Vietnam. The USA was afraid that communism would spread to South Vietnam and then the rest of Asia. It decided to send money, supplies and military advisers to help the South Vietnamese Government.