I Corps () 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 at the Vietnamese Demilitarized Zone (DMZ). These five provinces are Quảng Trị Province, (Khe Sanh, Đông Hà, Quảng Trị City), Thừa Thiên-Huế Province, (Phu Bai, Huế City), Quảng Nam Province, (Đà Nẵng, Hội An), Quảng Tín Province, (Tam Kỳ, Chu Lai) and Quảng Ngãi Province, (Quảng Ngãi).
History 1957-1961 I Corps headquarters became operational at Danang on 1 June 1957 and was responsible for the area from Danang north to the DMZ. US advisors at I Corps headquarters consisted of about a dozen officers and enlisted men with a colonel of infantry as senior corps adviser, and two lieutenant colonels to advise the corps engineer, armor, ordnance, and signal units. Its assigned units were the 1st Field Division at Danang and the 2nd Field Division at Huế. US advisors at I Corps headquarters consisted of about a dozen officers and enlisted men with a colonel of infantry as senior corps adviser, and two lieutenant colonels to advise the corps engineer, armor, ordnance, and signal units.
I Corps' operational strategy focused its efforts on the heavily populated coastal plain, moving gradually westward to secure the piedmont. The bulk of the mountainous, but sparsely populated interior remained in VC hands, punctuated only by occasional raids to keep them off balance. The staff planned to establish a chain of strategic hamlets running north to south to block VC penetrations to the coast. The only new feature was a series of major clear-and-hold operations moving progressively from the southern end of the corps north to the DMZ.
1962 In January, the United States Army's 93rd Transportation Company (Light Helicopter), equipped with H-21C Shawnees was deployed to Da Nang Air Base.
In early September 1962, the United States Marine Corps (USMC) Operation Shufly helicopter squadron, equipped with HUS-1s (UH-34s) began redeploying with its support units from Soc Trang Airfield in IV Corps to Da Nang Air Base, completing the redeployment by 20 September. Simultaneously, the U.S. Army's 93rd Helicopter Company moved from Da Nang to Soc Trang.
1964 Following the 1963 South Vietnamese coup d'état, coup leader General Dương Văn Minh reassigned II Corps commander General Nguyễn Khánh to command I Corps, allegedly to get him as far from Saigon as possible. On 30 January 1964, Khánh overthrew Minh in a bloodless coup. By 6 March, Khánh had replaced three of the four Corps commanders. Khanh appointed BG Tôn Thất Xứng, to lead I Corps.
On 24 February 1964, Khánh issued the 1964 National Campaign Plan under the name Chien Thang (Struggle for Victory). Khanh decided the provinces surrounding Saigon would receive top priority in the distribution of troops, civil servants, and money. The rest of III Corps and IV Corps were next in the resource queue, whereas the provinces of II and I Corps had the lowest priority. One reason why the north received the least resources was that, at least before late 1963, it had appeared to be in the best shape. Thus, Chien Thang forecast that I and II Corps would be the first to enter into the final phase, the destruction of the enemy's last major formations and bases in January 1965, whereas III and IV Corps would not reach that point until January 1966.
The Diem regime's relative success in pacifying significant parts of I and II Corps had led to decisions in 1963 and 1964 to transfer troops to more troubled areas further south. This meant that troops were leaving just as the VC were increasing their efforts. The number of VC-initiated actions per month in I Corps quadrupled between November 1963 and February 1964. By infiltrating small guerrilla propaganda and terror teams from the mountains into the coastal plain, the VC began seriously to erode government control in certain areas.
I Corps' southernmost provinces of Quang Tin and Quang Nam posed the most difficulties. The French had never truly controlled this area during the Indochina War, handing South Vietnam a difficult legacy. The majority of enemy forces in I Corps resided in these two provinces, with each hosting a VC regimental headquarters and several battalions. The 2nd Division had responsibility for these provinces. In December 1963, the division had begun a new round of patrolling and some intensive training for an even greater effort to come. The plan for Quang Tin and Quang Nam envisioned a multiphase, 12-month process. During the first phase, from February to May, the ARVN would secure a narrow strip along Highway 1. In subsequent phases, the Civil Guard, protected by the 2nd Division's dense patrol screen, would solidify control over first the eastern coast and then the piedmont. The final phase, the destruction of the VC's mountain bases, would begin in 1965. During the first two phases, regimental commanders would assign battalions and companies to discrete geographical areas for the conduct of extensive day and night platoon- and company-size patrols. After a battalion cleared its zone, a task the Vietnamese estimated would take about three weeks, it would transfer the burden of holding the region to the Civil Guard and move to clear a new area, so that eventually government control would spread to all populated areas. The strategy saw initial success with Operation Dan Chien I reconquering the Phuoc Chau valley and returning 11,000 civilians to government control by late April.
On 24 March, a 53-man platoon from the 1st Battalion, 9th Marines arrived at Da Nang Air Base and assumed responsibility for security around the USMC compound and flightline.
By mid-1964, the US Corps senior advisor reported that "The pacification of I Corps Tactical Zone... is progressing inexorably towards the final victory over the Viet Cong."
In early July, as part of their summer offensive the VC and People's Army of Vietnam (PAVN) units attacked Nam Dong, losing the battle but forcing the relocation of the base. The 1st Division had its hands full trying to prevent pacified areas from regressing during the summer. Most operations inflicted few casualties, as the VC broke into small groups and avoided contact. The government worsened the situation by diverting all recruits meant for I and II Corps to III and IV Corps. In mid-August, the 2nd Division's new commander, Colonel Nguyen Thanh Sang, abandoned his predecessor's method of saturation patrolling. He terminated Operations Dan Chien 1 and 2 in favor of a more passive posture. As a result, pacification regressed in the Corps over the summer. Significant declines in population control in the 2nd Division's area, particularly in Quang Nam and Quang Tin Provinces, more than offset small gains in the 1st Division zone. By September, I Corps adviser Colonel Wohner admitted that the situation was "not encouraging."
In September, the Joint General Staff (JGS) reassigned Quang Ngai province from II Corps to I Corps.
In mid-September, four USMC four-man advisory teams drawn from the 3rd Marine Division, deployed to the Corps and were assigned to 1st and 2nd Division units.
On a visit to I Corps in October COMUSMACV General William Westmoreland described the situation as "depressing" and "dismal." Another officer noted that Highway 1 "is interdicted daily and must be traveled with armed escort. Almost all piedmont areas and many parts of the coastal plains are under communist control, and government administration below the district level is practically non-existent."
In November in the aftermath of the September 1964 South Vietnamese coup attempt, Khánh appointed Brigadier general Nguyễn Chánh Thi as the new Corps commander.
On 26 November with the rotation of the USMC security unit at Da Nang Air Base, Company L, 3rd Battalion, 9th Marines was reinforced with engineers, 81mm mortar teams, and counter-mortar radar personnel and designated as the Security Detachment, Marine Unit Vietnam.
By the end of the year, US advisers acknowledged that pacification had "made little progress, if any." All of I Corps' provincial advisory teams considered the state of pacification as unsatisfactory. US officers attributed this state of affairs to the enemy's growing strength, the result of increased recruiting and, particularly worrisome in their minds, increased North Vietnamese infiltration. One Marine officer stated "If this input is continued, we will shortly be in the third phase of counterinsurgency," that is, mobile conventional warfare. Population control was assessed to have declined from 81% in January to 51% in December.
At the end of 1964, a US Army helicopter aviation company or USMC helicopter squadron was assigned in direct support of each Vietnamese infantry division. Further fixed-wing transport, reconnaissance, and observation aircraft were available as well. As a result, each senior Corps adviser had between 70 and 100 aircraft at his disposal, with MACV retaining control over the rest.
1965
Military Region 5 prioritized I Corps second to II Corps for its winter-spring offensive. It expected its forces in I Corps to continue to erode the government's presence and, if possible, to defeat a few government units. Most importantly, it directed its troops in southern I Corps to assist those in northern II Corps in cutting South Vietnam in two. As for the government, its plans for I Corps focused on furthering pacification, continuing its long-standing effort to secure the populated flatlands that lay between the South China Sea to the east and the north–south rail line to the west. The two visions were incompatible.
Thi embraced the government's pacification plan except in Quang Nam and Quang Tin, where he felt it was more important to pacify the piedmont immediately west of the railroad before moving east to the coast. He allocated 70% of his regular army forces to pacification and security work. In all provinces, he assigned primary importance to keeping Highway 1 open.
The VC didn't undertake any major attacks in January, but continuous low-level subversive activity and distractions caused by political turmoil in Saigon and Buddhist riots in several northern cities meant the Corps made little progress in pacification. Nor would it make any progress the following month, for as in II Corps, the VC launched a fresh wave of assaults in the early hours of 7 February attacking an outpost protecting Tam Ky losing 230 killed and seven captured while killing 37 ARVN. The attacks continued across the Corps until late March.
On 7 February, as US aircraft conducted Operation Flaming Dart, bombing attacks on North Vietnam in retaliation for the attack on Camp Holloway in II Corps, President Lyndon Johnson ordered the USMC to deploy an antiaircraft unit to protect Da Nang Air Base, with the 1st Light Anti-Aircraft Missile Battalion arriving on 8 February. When Johnson opted to replace Flaming Dart's tit-for-tat retaliatory strikes with Operation Rolling Thunder in late February, a violent reaction from North Vietnam now deemed more likely. As a result, on 8 March the 9th Marine Expeditionary Brigade (9th MEB) went ashore to defend Da Nang Air Base.
During the first three months of 1965, the percentage of the Corps' more than 1.8 million people living under government control had risen from 24% to 26%. Similarly, the percentage of the population over which the government either controlled or exercised some manner of influence had risen from 51% to 56%. As in II Corps, the increases were due as much to people fleeing the VC as to successful pacification operations. By the end of March 1965, 112,000 refugees crowded into I Corps' towns and cities.