Final answer:
The United States supported Ngo Dinh Diem in South Vietnam during the Cold War as a potential ally against communism. However, Diem's autocratic and oppressive rule led to his loss of popularity and assassination.
Step-by-step explanation:
The United States supported Ngo Dinh Diem in South Vietnam because they saw him as a potential ally in their fight against communism during the Cold War. Initially, the United States hoped that Diem could create a stable and democratic government in South Vietnam that would serve as a counterbalance to the communist government in North Vietnam. However, Diem's regime became increasingly autocratic and oppressive, which led to his loss of popularity among the South Vietnamese people and eventually his assassination.