During the Vietnam War, South Vietnam was divided into four Corps Tactical Zones, also called Military Regions, and the Special Capital Zone (Saigon area) for purposes of military operations. The four Corps Tactical Zones were identified as I-Corps (pronounced as "EYE"), II-Corps, III-Corps and IV-Corps.  Each Corps was an administrative and command area for tactical operations.  I-Corps was located in the region nearest North Vietnam and adjacent to the DMZ.

The DMZ (demilitarized zone) between North and South Vietnam was established in April, 1954 as a result of the Geneva Conference ending the war between the Viet Minh and the French.  The DMZ in Vietnam lay at the 17th parallel north.  In reality, the DMZ extended about a mile on either side of the Ben Hai River and west to east from the Lao border to the South China Sea