The 21st Division of the Army of the Republic of Vietnam (ARVN)—the army of the nation state of South Vietnam that existed from 1955 to 1975, was part of the IV Corps that oversaw the southernmost region of South Vietnam, the Mekong Delta.
The 21st Division was based in Bac Lieu, the southernmost division in the whole country, in an area dominated by jungles and swamps.
References
21st Division (South Vietnam) Wikipedia(Text) CC BY-SA