The PAVN objective was to take control of Quảng Trị Province, isolate the city of Huế and, if the opportunity presented itself, capture the entire Tri Thien-Huế area.

[10] In the final phase of their operation, the PAVN and VC would cut off Da Nang from the surrounding regions which made up I Corps Tactical Zone, and prevent reinforcements from retaking the city.

Those forces were equipped with 449 tanks and armored vehicles, 418 artillery pieces, 64 M42 Duster light air-defense weapons, 373 aircraft of different types, and 165 naval vessels of different sizes.

[12] In combination with the ‘Ly Thuong Kiet Military Plan’ and experiences gained during the North Vietnamese Easter Offensive, Trưởng organised the defenses of I Corps into three phases, to be staged from three different areas.

The opening shots of the campaign was marked by an attack on South Vietnamese military convoys on the Hai Van Pass and the destruction of An Lo Bridge on Highway 1, located north of Huế.

Positions held by the ARVN 913th Regional Force at Dong Ong Do and Hill 368 also came under attack, as Phu Bai Airbase was subjected to intense artillery bombardment.

Over the period of one week, both sides fired over 8,000 rounds of artillery shells on Hill 224, and the RVNAF conducted over 60 bombing sorties in an attempt to halt the advances of the PAVN 2nd Corps.

[19] On the following day, Đồng's forces staged their attacks from two main directions, from Thanh Hoi and Tich Tuong-Nhu Le, moving along Route 68 and National Highway 1 respectively.

ARVN Colonel Do Ky, also the provincial chief of Quảng Trị Province, tried to lead what was left of his troops back to Huế but was pursued by the North Vietnamese along National Highway 1 until they reached An Lo.

As the fighting in Quảng Trị unfolded, Trưởng flew back from Saigon where he tried to obtain approval from Thiệu for his new defensive plan, and hastily reorganised South Vietnamese defences in the northern regions of I Corps.

However, as the main roads were blocked by the PAVN, Thi's only option was to use a coastal corridor between Thuận An and Tu Hien, and board transport vessels belonging to the 106th Navy Squadron and head back to Da Nang.

At 16:30 on March 23, the PAVN 101st Regiment, 325th Division overran Luong Dien and surrounded Phu Bai Airbase, to close the main corridor leading into Huế from the south.

Recognising that he no longer had the manpower to mount a counter-attack, Nhựt ordered his troops to abandon Tra Bong and Son Ha, and concentrate his units in Tam Kỳ and Chu Lai instead.

[30] At around midday, Nhựt ordered the rest of the 4th Infantry Regiment out from Quảng Ngãi in an attempt to hold Tam Kỳ, so South Vietnamese defences in that province was further weakened.

[30] In Quảng Ngãi, the PAVN 52nd Independent Brigade, with support from two special forces battalions as well as tanks and armoured vehicles from the 574th Regiment, rolled over South Vietnamese defences.

It was a major economic and political centre in I Corps, and was home to the largest military installations which incorporated the South Vietnamese army, navy and air force.

On March 24, the PAVN 325th Division fought with the 258th Marine Brigade and the 914th Regional Force Battalion at north Hai Van, and overran Phuoc Tuong, Nuoc Ngot, Tho Son and Thua Luu.

[37] Meanwhile, the 369th Marine Brigade quickly abandoned their position at Son Ga, after they had detected elements of the PAVN 304th Division conducting reconnaissance missions on the battlefield.

And in addition to the South Vietnamese navy vessels of I Corps, U.S. General Homer D. Smith also provided five barges, six passenger liners and three cargo ships to help with the evacuation process.

[43] Even though the evacuation plan was designed to be carried out in an orderly manner, chaos and confusion quickly descended on the civilian and military population of Da Nang as people fought each other to board the vessels anchored off the coast.

Thiệu's decision to abandon the Central Highlands and the coastal regions of I Corps had a severe negative impact on ARVN morale, especially as the northern provinces of South Vietnam were driven into chaos.

On March 31, after the fall of Huế and Da Nang, ARVN General Phạm Văn Phú, commander of II Corps Tactical Zone, held a meeting with his General Staff and the provincial chiefs of Bình Định, Khánh Hòa, Phú Yên, Ninh Thuận and Bình Thuận and requested them to form a defensive line from Qui Nhơn into the Mekong Delta to cover the retreat of the ARVN from the Central Highlands.

Meanwhile, General Phan Dinh Niem, the commander of the ARVN 22nd Infantry Division, was required to form a defensive line linking Qui Nhơn with Diêu Trì and Deo Ca, with the purpose of delaying the PAVN.

The destruction of the ARVN 22nd Infantry Division, which had about 10,000 fully equipped men, caused chaos in Nha Trang; more than 3,000 South Vietnamese recruits from the Lam Son Army Training Centre retreated into the city, and another 1,000 soldiers went on a rampage and looted goods on the main streets.

The chief of Khánh Hòa Province, Colonel Ly Ba Pham, boarded a military aircraft and flew out to Phan Rang Air Base after he had notified Saigon that "the situation is irreversible".

[50] According to many academics, both in Vietnam and in the West, the major factor which contributed to the rapid collapse of South Vietnamese defences in 1975 was the numerous and contradictory orders issued by Thiệu.

[51] During the various phases of the South Vietnamese retreat from the northern provinces, namely in I and II Corps Tactical Zones, Thiệu had at least three different plans and decisions laid out before him, and all lacked consistency or simply contradicted one another.

On March 17, with intervention from General Cao Văn Viên, Thiệu contradicted his previous orders when he approved Trưởng's defensive plan to hold the major cities of Huế and Da Nang.

[53] However, the very next day, Thiệu ordered Trưởng and General Thi to abandon Huế and to concentrate only sufficient numbers of South Vietnamese units to hold Da Nang.

[56] For example, on April 2, Thiệu requested the country's National Assembly to dissolve Prime Minister Trần Thiện Khiêm's cabinet and to replace him with Nguyễn Bá Cẩn.