The correct answer is D) Richard Nixon beliefs about Communism.
The event that did not protect South Vietnam from North Vietnam was Richard Nixon beliefs about Communism.
United States President Richard Nixon had a different approach to the War of Vietnam than its predecessor Lyndon B.Jhonson. Nixon thought that South Vietnam had the total responsibility to defend its liberty from its aggressor, North Vietnam. So the United States role changed, he began to order the gradual retire of U.S. troops from South Vietnam and supported the Asian country with other types of things. So it's true that The event that did not protect South Vietnam from North Vietnam was Richard Nixon beliefs about Communism.