Giap chose to first challenge the French along the Black River, where seven French bases dotted the banks reaching south from the Red River. La Phu, the northernmost, lay on the west bank, followed by Dan The, Ap Da Chong, Xom Bu, Ap Phu To, and Rocher Notre-Dame on the east, and finally Tu Vu, across from Rocher Notre-Dame on the west. Given the sheer immensity of the mountain chain looking down on the lower reaches of the river from the west, French strategy for controlling the Black River had to be anchored on controlling both the east bank and the critical west bank toehold at Tu Vu. From those positions, French artillery and air could be directed against any Viet Minh forces threatening the passage of riverine resupply elements. Their problem was that the terrain commanding the east bank between Dan The and Rocher Notre-Dame was only slightly less dangerous for defensive forces than that lying across the river. From Dan The to Ap Da Chong, the Ba Trai forest and irregular terrain stretched east to Yen Khoi and Yen Cu. South of the Yen Cu–Ap Da Chong road, Ba Vi Mountain and nearby peaks rose to 4,252 feet. Between those and a hill directly east of Rocher Notre-Dame rising to 2,326 feet lay the Xom Sui depression. South of Rocher Notre-Dame a narrow strip of swamp, sandbars and rice paddies, cut by rivers, pushed out a few kilometers east to meet the forest until it neared Cham Mountain, from where it entered the Colonial Route 6 sector and rose to 1,332 feet. Sitting astride the Black River, Dodelier's Mobile Group 7 had four infantry battalions manning strongpoints at Rocher Notre-Dame, Tu Vu, Dan The and La Phu, backed up by the 1st Colonial Paratroops acting as sector reserve out of Ap Da Chong.

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By December 4, elements of Colonels Thao's and Tan's Divisions had moved into positions from where they could cut off the 1st BPC at Ap Da Chong. Captured Viet Minh officers told the French that their first step would be the isolation of Ap Da Chong, after which they would attack lines of communication and strongpoints. Two regiments of the 312th Division, identified in the area east of Rocher Notre-Dame, gave credence to this threat.

The mounting Viet Minh presence in the area prompted the Dodelier group to reinforce Thu Phap with elements from the 1st BPC while launching a spoiling attack to ease the threat against Rocher Notre-Dame. Colonel Thomazo's Mobile Group 4 carried out this attack on December 9, 1951, in conjunction with sector troops and Major Moulie's 7th BPC. Their group was to clear the southern slopes of Ba Vi Mountain and the route between Chai Koai and Thuy Co.

By December 10, Thomazo's men were pushing in from Van Mong and Yen Le to penetrate the area near Mount Ba Vi. This attack, however, bogged down in heavy terrain, prompting a covering group from Task Force North to undertake the principal assault. Major Moretti's 1st Colonial Parachute Battalion, moving east, ran into four to five battalions from Regiment 209 in Xom Sui depression. The fighting proved vicious and was soon hand to hand. Only close air support from fighter-bombers allowed the 1st Colonial Paras to break contact. Regiment 209 pulled back its units in recognition of French air superiority, but the 1st BPC was able to recover only 15 bodies. Eighty-seven paras were missing in action. Their sacrifice forced Colonel Tan to cancel his attack against Rocher Notre-Dame.

Viet Minh plans, however, had called for a simultaneous attack against both Rocher Notre-Dame and Tu Vu, and the latter part of the plan had not been derailed. Heavy mortars rained fire on Tu Vu at 2130 that night, followed by human-wave attacks. Under the flicker of parachute flares, two companies of Moroccan tirailleurs and a platoon of tanks beat back wave after wave of Viet Minh. By 0340 on the morning of December 11, elements of Regiment 88 had penetrated the inner perimeter. Under cover of the tanks, which were all lost in subsequent fighting, the Moroccans withdrew to a sandbar in the middle of the river while three artillery batteries on the Rocher Notre-Dame side poured fire into the post. At sunrise what was left of Regiment 88 withdrew, leaving 250 bodies in the wire and another 150 bodies scattered throughout the area. A smaller attack against a single battery firebase at Xom Bu was also driven off.

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