Dong Ha River,Hieu Giang River "song Hieu Giang" (DongHa City.)

Exactly at noon on Holy Thursday an incoming artillery round hit the 3rd Division,s headquarters at Ai-Tu Combat Base just north of Quang Tri. There was something new in the sound. Under cover of the artillery barrage, 30,000 North Vietnamese troops and more than 200 tanks, the equivalent of three division, charged across the demilitarized zone that had been established sixteen years before in Geneva. the North had cast prudence aside and in so doing had confounded American and South Vietnamese assumptions that the enemy would not risk so overtly violating the DMZ. On Easter morning, April 2nd, two North Vietnamese columns of tanks-both amphibious PT76 and T54 main battle tanks,and supporting infantry troops approached the Highway 1 bridge at Dong Ha (this bridge) fourteen kilometers below the DMZ. Slowing the enemy columns with a number of direct hits. The marines deployed on the south side Dong Ha Bridge trying to stop the enemy tanks by the blew up the bridge and dropped into the river by TNT and C-4 explosives.The bridge had been repaired.

On 1 April, General Giai, ordered a withdrawal of the 3rd Division south of the Cua Viet River in order for his troops to reorganize. The following morning, ARVN armoured elements held off a PAVN attempt to cross the river at Dong Ha by destroying the bridge there.[30] The initial PAVN units were then joined by the 320B and 325C Divisions. Simultaneously, the 324B Division moved out of the A Shau Valley and advanced directly eastward toward Fire Bases Bastogne and Checkmate, which protected the old imperial capital of Hue from the west.

The North Vietnamese advance had been timed to coincide with the seasonal monsoon, whose 500 feet (152 m) cloud ceilings negated many U.S. airstrikes.[31] PAVN advance elements also soon followed by antiaircraft units armed with new ZSU-57-2 tracked weapon platforms and man-portable, shoulder-fired Grail missiles, which made low-level bombing attacks hazardous.
North Vietnamese T-54 tanks

Camp Carroll, an artillery firebase halfway between the Laotian border and the coast, was the linchpin of the South Vietnamese northern and western defense line and was the strongest obstacle to the North Vietnamese before Quang Tri City. On 2 April Colonel Pham Van Dinh, commander of the 56th ARVN Regiment, surrendered the camp and his 1,500 troops with barely a shot being fired.[32] Later in the day, ARVN troops abandoned Mai Loc, the last western base. This allowed North Vietnamese forces to cross the Cam Lo bridge, 11 kilometers to the west of Dong Ha. PAVN then had almost unrestricted access to western Quang Tri Province north of the Thach Han River.

The PAVN advance was slowed by delaying actions for three weeks, and the South Vietnamese launched several counterattacks, but on the morning of 27 April, the North Vietnamese came on again, launching multi-pronged attacks against Dong Ha (which fell on the following day) and advancing to within 1.5 kilometers of Quang Tri City. General Giai had planned a staged withdrawal from the city to consolidate south of the Thach Han, but bewildered by conflicting orders from Lam and Giai, most ARVN formations splintered and then collapsed, conceding most of the province north of the city.[33] On 29 April, Giai ordered a general retreat to the My Chanh River, thirteen kilometers to the south. U.S. military advisors in Quang Tri called for emergency helicopter extraction and, on 1 May, 132 survivors were evacuated from Quang Tri, including 80 U.S. soldiers.

The exodus of ARVN forces was joined by tens of thousands of South Vietnamese civilians fleeing from the fighting. As the mass of humanity jostled and shoved its way south on Highway 1, it presented an inviting target for North Vietnamese artillerists.[34] They were soon joined by PAVN infantry, who moved by the flank to attack the column. ARVN units, with no leadership and all unit cohesion gone, could muster no defense. Meanwhile, to the west, Fire Support Bases Bastogne and Checkmate had fallen after staunch ARVN defense and massive B-52 strikes, which inflicted heavy casualties.[35] On 21 April Abrams notified the U.S. Secretary of Defense that

In summary...the pressure is mounting and the battle has become brutal...the senior military leadership has begun to bend and in some cases to break. In adversity it is losing its will and cannot be depended upon to take the measures necessary to stand and fight.[36]

Giai evacuated the last of his forces from Quang Tri City, which fell to PAVN forces on 2 May. That same day General Lam was summoned to Saigon for a meeting with President Nguyen Van Thieu. He was relieved of command of I Corps and replaced by Lieutenant General Ngo Quang Truong, commander of III Corps and one of the ablest ARVN generals.[37] Truong's mission was to defend the old imperial capital of Hue, minimize further losses, and retake captured territory. Although saddled with raw troops and constantly countermanded by his superiors, General Giai had conducted a reasonably good defense. Even Truong pleaded his case with Thieu, wanting to keep Giai in command of the 3rd Division.[38] It was in vain. Giai, who was to be made the scapegoat for the collapse, was tried for "desertion in the face of the enemy," and sentenced to five years in prison.[39]