On December 11 and 12, as Mobile Group 4 inched toward Rocher Notre-Dame, the Viet Minh changed their strategy. Rather than continuing attacks against French strongpoints, with their interlocking bands of defensive fire, mines, and artillery and air support, they decided to hit French lines of communication, thereby choosing the terrain on which they would engage French forces. To this end, Tan ordered Regiments 165 and 209 to infiltrate to positions in the Ba Trai and an area north of Ba Vi. Thomazo’s Mobile Group 4 soon ran into heavy Viet Minh elements manning a series of cuts in the road between Yen Chu and Ap Da Chong. As at Xom Bui, the French called on close air support, but this time the Viet Minh held.

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An attack by the 5th Colonial Paratroops (5th BPC), under Major Orsini, backed up by a squadron of Sherman tanks from the Far Eastern Colonial Tank Regiment (RBCEO) also failed to dislodge them. By nightfall on December 12, the road remained blocked. Thomazo ordered Orsini to pull the 5th BPC back, but breaking contact proved to be difficult. A battalion from Regiment 165 caught one company in an ambush, killing 34 paratroops and wounding 66. Three of the dead had been platoon leaders.

On December 13 and 14, the 312th Division’s pressure relaxed. Thomazo pulled Mobile Group 4 back from Rocher Notre-Dame and advanced north to clear the Trung Ha-Yen Khoi region while the high command reinforced Dodelier’s Black River sector with Mobile Group 1, Armored Subgroup 1, the 1st Foreign Legion Paratroops, and the 1st Tank Battalion of the RBCEO. In conjunction with Thomazo’s Mobile Group 4, these forces were to conduct an envelopment operation designed to clear the 312th Division from the Ba Trai region and off the western slopes of Ba Vi mountain.

Colonel Dodelier decided to focus his initial efforts on clearing the Viet Minh forces in the Ba Trai forest. To do so, his forces needed to reach the Ap Da Chong to Dan The bypass while maintaining control of the road between Yen Cu and Ap Da Chong. This would cut off the 312th Division’s retreat across the Black River or south into the Ba Vi Mountain area. To block their northern escape, he assigned Armored Subgroup 1, reinforced with a battalion from Mobile Group 4, to blocking positions on the road between Cam Dai and Dan The. The task of controlling the road between Yen Cu and Ap Da Chong was assigned to a Paratroop Task Force under Lt. Col. Ducournau consisting of the 1st BEP, the 5th BPC and a platoon of tanks from the 1st Battalion, RBCEO. The North Africans of Colonel Edom’s Mobile Group 1 were to clear the road along the eastern route into the Ba Trai.

The northern and southern blocking forces took up their initial positions on December 15. As Edom’s Mobile Group cleared the eastern approaches into the Ba Trai forest, the unit ran into elements of the 312th Division near Xom Doi. The North Africans drove the Viet Minh off, only to run into them again at Hill 116, one kilometer northwest of Xom Doi. The 312th Division fought well, but superior French firepower and air support carried the day. By that evening, Mobile Group 1 had taken the heights.

Elements from the Black River sector pushed on to reach the hills overlooking the Black River, while Ducournau’s paratroops pushed westwards on the road leading to Ap Da Chong. By nightfall, the paratroops controlled the road. One firefight, requiring air and artillery support, left a Frenchman dead and 26 wounded. Viet Minh movements throughout the night prompted French Intelligence to believe that the 312th Division was withdrawing west of the Black River.

With the Ba Trai forest temporarily cleared and French blocking forces in position, Dodelier turned his attention to the western slopes of the Ba Vi, where Viet Minh forces still threatened Rocher Notre-Dame. On December 17, the French launched another pincers attack. This time a task force composed of the 1st BEP, the 2nd Battalion, 6th Moroccan Tirailleurs (2/6th RTM), and the 2nd Battalion, 1st Algerian Tirailleurs (2/1st RTA), launched an attack from west of Ba Vi Mountain to clear the Lang Gy depression. while the 2nd Foreign Legion Parachute Battalion (2nd BEP) moved south from the Ap Da Chong–Yen Cu road to clear the heights under cover of the 3rd Battalion, 4th Moroccan Tirailleurs (3/4th RTM) and 5th BPC. Linkup between the two task forces took place without a hitch at 1500, leaving the 1st BEP, 5th BPC, and 3/4th RTM in control of the road to Ap Da Chong. While the 2nd BEP took up positions on Hill 564, Edom’s Mobile Group, along with Armored Subgroup 1, were withdrawn from the sector. Dodelier’s remaining task was to re-establish contact with the southern strongpoints, which had been cut off since December 11.

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