I Corps tactical zone

During the Vietnam War, the Army of the Republic of Viet Nam (ARVN) defined I Corps tactical zone (I CTZ) as the five northernmost provinces of South Vietnam: from approximate north to south, these were

Provinces in I CTZ

1965 name	Capital	Current name
Quang Tri	Dong Ha	Quang Tri
Thua Thien	Hue	Thua Thien-Hue
Quang Nam	Tam Ky	Quang Nam
Quang Tri	Dong Ha	Quang Tri
Quang Ngai	Quang Ngai	Quang Ngai

Its northern border was the Demilitarized Zone (DMZ) with North Vietnam. On the west, it bordered Laos and areas of the Ho Chi Minh trail.

The largest cities of the zone were Hue and Danang. Major engagements, at small locations, included the Battle of Khe Sanh and the Battle of Con Thien.

North Vietnamese equivalents

These areas came under Military Regions 4 and 5, as well as the B-5 Front.

U.S. equivalents

American forces in I Corps began with III Marine Amphibious Force, its name changed from the usual designation of III Marine Expeditionary Force due to "expeditionary force" having unfortunate connotations, to the Vietnamese, from French usage. Later in the war, the Military Assistance Command, Vietnam, asserted more control over the Marines, variously putting a forward echelon of MACV there, and establishing TASK FORCE OREGON and Provisional Corps, Vietnam.