I Corps ( South Vietnam ) The I Corps Tactical Zone ( ) was a corps of the Army of the Republic of Vietnam ( ARVN ) , the army of the nation state of South Vietnam that existed from 1955 to 1975 . It was one of four corps of the ARVN . This was the northernmost region of South Vietnam , bordering North Vietnam . These five provinces are Quảng Trị Province , ( Khe Sanh , Đông Hà , Quang Tri City ) , Thừa Thiên - Huế Province , ( Phu Bai , Hue City ) , Quảng Nam Province , ( Đà Nẵng , Hoi An ) , Quang Tin Province , ( Tam Ky , Chu Lai ) and Quang Ngai Province , ( Quang Ngai ) . Among its formations and units were the 1st Division . The I CTZ , later Military Region 1 , was partnered with the U . S . III Marine Expeditionary Force and the XXIV Corps . Lam Son 719 . General Hoang Xuan Lam ( Hoàng Xuân Lãm ) was given responsibility for the I Corps Tactical Zone in 1967 . He coordinated the South Vietnamese Operation Lam Sơn 719 offensive which aimed at striking the North Vietnamese logistical corridor known as the Hồ Chí Minh Trail in southeastern Laos during 1971 . Due to his political connections with President Nguyễn Văn Thiệu , he was still serving as I Corps commander when the North Vietnamese launched the " Nguyên Huế Offensive " ( called the Easter Offensive ) in 1972 . Lãm was recalled to Saigon on 2 May 1972 by Thiệu , who relieved him of his command , due to complaints regarding Lãm ' s fitness and competency as a general . He was succeeded as commander by Ngo Quang Truong . 1975 Spring Offensive . I Corps disintegrated during the 1975 Spring Offensive ( the Hue – Da Nang Campaign ) . The situation for the South Vietnamese in the I Corps Tactical Zone had regained some stability after the defeat of a three - division PAVN push during late 1974 . By early the following year , I Corps fielded three infantry divisions ( the 1st , 2nd , and 3rd ) , the elite Airborne and Marine Divisions , four Ranger Groups and the 1st Armored Brigade . Until mid - March , the North Vietnamese had limited their offensive operations to attempts to cut Highway 1 , the main north / south line of communication , between Huế and Da Nang and between Da Nang and Chu Lai . To confront the South Vietnamese , PAVN Brigadier General Le Trong Tan had amassed a force of the crack 2nd , 304th , 324B , 325C , and 711th PAVN Divisions and nine independent infantry regiments , three sapper regiments , three armored regiments , twelve anti - aircraft and eight artillery regiments . At a meeting in Saigon on 13 March President Thieu was briefed on the military situation by Truong and another corps commander . Thieu then laid out his plan for national consolidation . As Truong understood it , he was free to redeploy his forces to hold the Da Nang area . Truong was shocked to discover , however , that the Airborne Division was to be removed to III Corps ( unknown to Truong at the time , the Marine Division was also already earmarked for redeployment with both units then forming a new national reserve ) . General Truong was recalled to Saigon on 19 March to brief Thieu on his withdrawal plan . The general had developed two contingency plans : The first was predicated on government control of Highway 1 , which would be utilized for two simultaneous withdrawals from Huế and Chu Lai to Da Nang ; The second course presupposed PAVN interdiction of the highway and called for a withdrawal into three enclaves : Huế , Da Nang , and Chu Lai . This was to be only an interim measure , however , since the forces that withdrew to Huế and Chu Lai would then be sea - lifted to Da Nang by the navy . The president then stunned the general by announcing that he had misinterpreted his previous orders : The old imperial capital of Huế was not to be abandoned . Making matters worse , Truong discovered that his force was to be reduced by the removal of the Marine Division .