Republic of Viet Nam
You probably mean who were the presidents of RVN during the war; Diem then later Thieu.
South (RVN-Republic of South Vietnam)
War in RVN (South Vietnam) since 1955. War with North Vietnam since 1964.
To preserve the RVN from a communist take-over.
Preserve the RVN from communist aggression.
RVN (Republic of South Vietnam); which no longer exists.
Fairly broad question. But we'll try this one; the Viet War went from a guerrilla war in RVN to a full scaled (minus nukes/invasion) conventional war against North Vietnam with the continued guerrilla & conventional war in RVN. Said another way: Instead of just one guerrilla war in one country (RVN) now there were two wars (guerrilla & conventional) involving two countries N & S Vietnam. RVN=Republic of South Vietnam.
War started in RVN in '55. War started against N. VN in '64.
To stop communist aggression; and preserve the RVN.
Mission: Stop communist expansion/preserve RVN
Stop communist aggression; and preserve the RVN.
'55-'64 RVN; '65-75 both RVN & North Vietnam.