Why did the united states support ngo dihn diem? a. he was anti-french. c. he was anti-socialist. b. he was anti-communist. d. he was anti-japanese.

The United States supported Ngo Dihn Diem because he was "anti-communist."  During the 1950s and early 1960s, the United States was greatly concerned that all of Vietnam would fall to communism.  North Vietnam was strongly supported by a communist government and also had the support of the communist Soviet Union. If South Vietnam was to fall into communism, the United States feared that all of Vietnam would turn communist and lead to other Asian nations slipping into communism and, in turn, placing the entire region under Soviet control.  Ngo Dihn Diem was the President of the Republic of Vietnam (South Vietnam) and was a vocal anti-communist, hence the U.S. supported him, feeling that his leadership would help to keep communism out of South Vietnam and keep such from taking over much of Asia.