1. 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.
2. War has become more gory and devastating. Advanced weapons have been invented and improved, such as atomic bombs. Conventional weapons have been abandoned; weapons with greater destructive power are used instead. Besides, political leaders have been accused of inhumane treatment of prisoners-of-war