Original sentence: The interim communist Republic of South Vietnam was established, until a unified Socialist Republic of Vietnam was inaugurated on July 2, 1976.US forces withdrew in 1973, and the South Vietnamese government surrendered to North Vietnam and its South Vietnamese NLF allies on April 30, 1975
Edited sentence: After the withdrawal of US forces,  North Vietnam and its South Vietnamese NLF allies completed its conquest of South Vietnam with the fall of  Saigon (now Ho Chi Minh City) and surrender of the South Vietnamese government on April 30, 1975.Upon surrender the communist backed Republic of South Vietnam took power until a unified Socialist Republic of Vietnam was inaugurated on July 2, 1976