After capturing the northern heights above Lạng Sơn, the Chinese surrounded and paused in front of the city in order to lure the Vietnamese into reinforcing it with units from Cambodia.

The Vietnam People's Army (VPA) high command, after a tip-off from Soviet satellite intelligence, was able to see through the trap, however, and committed reserves only to Hanoi.

Some Bangkok analysts stated Vietnam was at least successful in keeping their losses low by avoiding direct battles between its Hanoi based main-force units with the Chinese forces.

The Chinese planned to launch their assault along all of the five roads that led to Lạng Sơn from the north, east, and south (Highways 1A, 1B, 4A, and 4B and the railroad for China), filling each of the directions with at least two or three divisions.

[10] Waves of PLA troops from the 55th Army quickly overwhelmed Hill 386, a position situated 1.5 km south of the border, killing 118 Vietnamese.

[12] Though pockets of resistance were continued near the Friendship Pass and in Đồng Đăng, most of the Vietnamese defense was by then undertaken south of the town by the VPA 12th Regiment at the hamlet of Thâm Mô and its vicinity.

On 18 February, a Vietnamese counter-attack launched by the VPA 2nd Regiment drove PLA units back to their assemble positions on Hills 409, 611, and 675; this pattern was repeated on the following days.

[18] At Cồn Khoang, a village in the rear of Hill 339, the Vietnamese 63rd Company had fended off the assault of an unevenly outnumbering Chinese force on 17 February, virtually wiping out an enemy battalion.

[18] On 24 February, the VPA 1st Military Region Command officially established the Lạng Sơn Front with Major General Hoàng Đan in charge.

[19] Khau Ma Son, a strategic 800-meter height located 4 km west-northwest of Lạng Sơn that virtually obstructed all major roads near the town, fell to a Chinese unit at about 14:00.

Chinese control over Khau Ma Son made Vietnamese defense from Coc Chu to Hill 417 exposed to enemy's sight, as well as threatened VPA 12th Regiment's positions near Choc Vo.

[18] However, the Vietnamese reacted promptly, retreating from the Coc Chu – Hill 417 defensive line to a group of positions between Quán Hồ and Kéo Càng, thus blocking the highway access to Lạng Sơn once again.

The VPA command also reinforced their battered 12th Regiment by deploying the 42nd Regiment/327th Division to Kỳ Lừa, which was about 1 km north of Lạng Sơn, to face off the approaching PLA 43rd Army.

[20] As the Chinese attacked, they also managed to take control over all the roads connecting with Lạng Sơn, thus employing Mao Zedong's strategy of "the countryside surrounding the city" in order to avoid a Vietnamese trap of a "second Verdun".

During the period of 2–10 March, a company of the Chinese Unit 56037 lost one-third of its men as it was holding a hill near Bản Lan against successive Vietnamese assaults.

[23] While the battle for Lạng Sơn was taking its course, Vietnamese forces also carried out raids against Chinese border towns of Malipo and Ningming, but caused no significant losses.

[27] On 1 March, the PLA 165th Division/55th Army eventually pierced the Vietnamese defense held by the VPA 42nd Regiment at Lam Truong, which was 2 km northwest of Lạng Sơn.

The 163rd Division crossed the river between Khủi Khúc and Khòn Pát, penetrating 5 km south of Lạng Sơn to Pac Meng, a movement which had become the furthest advance toward Hanoi direction.