Operation "Castor" began on November 20, 1953 with the dropping of the 1st, 2nd and 6th Bns. colonials on the old airfield. Reinforced by two Bns. Equipped with 75 mm recoilless guns and a platoon of 81 mm mortars, they attacked the two Companies. Viet-minh who were there during training. In addition to these shock elements, the first parachuted group included a Co. of Gn. and a medical team which had been brought in 64 C-47s from Bac Mai and Gia Lam.
The French being in possession of the valley, the planes were able, from the 24th, to land on the aerodrome rehabilitated. In March, the garrison numbered 10,133 men:four Bns. of the Foreign Legion, the 3rd Bn. of Moroccan skirmishers, the 2nd Bn. of Inf. Vietnamese and the 3rd Bn. of Inf. Thai, 4 pieces of 155 mm, 24 of 105 mm, like also pieces ' AA of 20 mm and 40 mm intended either for the support on the ground, or for the engagement of the Chinese apparatuses which would have possibly had the intention to intervene. There was a small company. of transport totaling 127 vehicles and consisting of trucks, jeeps and ambulances, a Bn. of the Gn., two field hospitals, three supply units and a small aviation detachment comprising reconnaissance aircraft, 5 Curtiss SB2C Heildiver and Vought F4U Corsair fighter bombers, 4 C-47s and a Sikorsky S-51 helicopter for the evacuation of the wounded. It was a fine example of aerial resupply, culminating in the delivery of 10 M 24 Chaffee tanks which each required 5 C-47s and 2 Bristol Freighters to transport them. But the French had not decided whether they intended to establish a defensive position or a patrol base. In the end they achieved neither—although extensive networks of barbed wire had been erected, shelters and trenches were insufficient, and the patrols soon found they could not get far beyond the established defensive perimeter. . General Giap had taken up the challenge and his troops surrounded the garrison with a cordon of infantry, artillery and AA resources which would eventually suffocate him.
December 1953 to March 1954, Giap set up the 304th, 308th, 312' and 316' Div. of Inf. as well as the 351' Div. of Aie. and a Regt. of Gn. — in total:70,000 men. In addition, 60,000 auxiliaries of both sexes were put to work building roads and transporting supplies and ammunition. They also put in position 144 field artillery pieces (105 mm and 75 mm). 48 120mm mortars, 30 75mm recoilless guns and later 12 Katyusha sextuple rocket launchers. These were buried in proof positions, equipped with narrow embrasures, which constituted almost impossible targets for the French counter-battery to reach. On the hills, the Viet Minh also deployed more than 180 AA guns, ranging in caliber from 12.7mm to 37mm, which could lay down a denser barrage than those that had protected the Ruhr during World War II.
Giap had set D-Day for March 12, by which time he would have an eight-to-one advantage. Artillery opened fire on Diên Biên Phû on the 12th, and the following day two C-47s and a fighter were destroyed on the airfield.
At 5:15 p.m., the Viet Minh launched a strong attack against the 1st support point “Béatrice” which was held by the 3rd Bn. of the 13th Legion Demi-Brigade. Around 9:00 p.m., only an island of resistance was still holding, and after midnight it was silence:“Beatrice” had been overwhelmed and its garrison had suffered 75 percent losses, only 200 men having been able to evade the enemy.