Young people in South Vietnam was not so desperate toward the reunification

Without US South Vietnam was too weak.
After US forces withdrew, the defense of South Vietnam was too weak.
Why?

People in South Vietnam was so desperate as people in North Vietnam about the reunification of Vietnam.

I wonder young people in South Korea are desperate toward the unification of Korea.

http://www.dhahranbritish.com/history/A15_VietUnification.htm