The initial wave of the offensive was followed on 5 April by a PAVN advance out of Cambodia into Binh Long Province, northeast of Saigon. Its targets were the towns and airfields at Loc Ninh, Quan Loi, and An Loc. The possible initial goals of the offensive in III Corps remain unclear, but probably began as probes that, if successful, could be easily reinforced. The invasion was launched from Cambodian Base Area 708 by the B-2 Front's 5th PAVN/NLF Division and 203rd Armoured Regiment, which advanced down Highway 9 toward the border outpost of Loc Ninh. There, the 2,000 men of the ARVN 9th Regiment and a battalion of Rangers beat back five separate infantry/armor assaults before collapsing under the attack on 7 April. The North Vietnamese then isolated the 25th ARVN Division in neighboring Tay Ninh Province by sending two regiments to attack its forward outposts.
Sensing that the provincial capital of An Loc would be the next target, the III Corps commander, Lieutenant General Nguyen Van Minh dispatched the 5th Division to hold the town. They were reinforced by two battalions of the Ranger Group (on 7 April) and by two additional infantry battalions (on 10 April and 11 April). The 21st Infantry Division, which had been stationed in the Mekong Delta, was rushed to Chon Thanh to join a regiment of the 9th Infantry Division as a relief force. All forces in the area were placed under the command of Brigadier General Le Van Hung, commander of the 5th Division.
The move was fortuitous for the South Vietnamese, since communist forces were indeed proceeding eastward toward An Loc. Simultaneously, the 7th PAVN Division bypassed the town and moved south along Highway 13 to block any relief effort launched from Chon Thanh. The communists had decided that An Loc, with its close proximity to Saigon, would be proclaimed as the capital of the Provisional Revolutionary Government, but even if they had been able to seize the town, they would never have been able to hold it. American air power would have made such an eventuality impossible.
By 13 April, An Loc was surrounded and under a combined artillery, armored, and infantry attack by the 9th PAVN/NLF Division. North Vietnamese forces advanced on the town through a deluge rockets, bombs, and napalm delivered by U.S. and South Vietnamese aircraft supported by massed artillery, tank, and small arms fire. Inside the town, the contingent of U.S. advisors became essential to the defense, serving as a separate staff organizing fire and air support, logistics, and intelligence. Colonel William Miller, the senior U.S. advisor, was not happy with General Hung's continuous reluctance to launch counterattacks and his reliance on U.S. air power to defeat the North Vietnamese. His hesitation and lack of motivation prompted Miller to report that: "He is tired - unstable - irrational - irritable - inadvisable - and unapproachable."
The communist attacks persisted and PAVN forces eventually battered their way into the town, seizing the airfield and reducing the ARVN perimeter to about a square kilometer. During another assault on the 21st, PAVN tanks actually forced their way through the defense perimeter but were held at bay and then destroyed by anti-tank weapons and helicopter gunships. North Vietnamese infantry did, however, manage to seize most of the northern sector of the town, where they began digging in (often right across the street from the ARVN defenders). The initial shock of ARVN troops instilled by North Vietnamese armor was soon abated when they discovered that, because the supporting infantry failed to advance with the tanks, they became easy prey for anti-tank weapons. On other occasions, the opposite would occur, with massed infantry assaults moving forward without armored support. This failure of tactical coordination was one of PAVN's prime weaknesses during the offensive, and one that the allies were quick to exploit.
As a result of his failure to seize the town quickly, the commander of the 9th Division was officially reprimanded and local command was handed over the senior officer of the 5th PAVN/NLF Division. Besides the lack of coordination, the major difficulty for the North Vietnamese was the rain of ordnance delivered upon them by incessant air strikes, which further reduced manpower and made resupply difficult. After the failure of the assault on 21 April, the battle devolved into a siege, with the North Vietnamese pounding An Loc and its defenders with 1,200 to 2,000 mortar, rocket, and artillery rounds per day. An Loc was completely surrounded and could only be resupplied by air, a situation made more difficult by the loss of the airfield. Resupply was accomplished, however, by 448 aerial missions which managed to deliver 2,693 tons of air-dropped food, medical supplies, and ammunition, supported by the US Army's "549th QM (AD)" (QuarterMaster, Air Delivery) Rigger Company, which had been rapid-deployed from Okinawa in early April.
From 22 April to 10 May the tactical situation remained stable at what the Paris Match was calling "a Verdun or a Stalingrad" in III Corps. On the morning of 11 May, another North Vietnamese assault was launched after being preceded by an artillery bombardment that fired over 8,300 shells into a defense perimeter that had shrunk to a mere 1,000 by 1,500 yards before the day was over. PAVN forces again forced their way into An Loc, but the effort collapsed in the face of tremendous aerial attack which cost the North Vietnamese 40 tanks and over 800 men. The reasons for the failure were not hard to discern. Beginning at 05:30 that morning and continuing for the next 25 hours, the U.S. Air Force delivered a B-52 strike every 55 minutes to support the defense. For the next three days, each time PAVN troops assembled to resume the attack, they were bombed in their assembly areas.
A relief effort had been launched by the 21st ARVN Division, but it never arrived at An Loc. For three weeks the division crept northward along Highway 13 but it was held up by constant delaying actions by smaller PAVN forces. Although the division never reached its goal, it may have inadvertently saved the beleaguered city by eventually diverting almost all of the elements of the 7th PAVN Division from the fighting. The climactic attack on An Loc was launched on 14 May, when the North Vietnamese attacked directly into the teeth of the ARVN defense. The failed assault was described by Colonel Walt Ulmer, the 5th Division's senior advisor: "they were simply trying to pile on and pile on and pile on. They frittered away an awful lot of manpower."