This is coolbert: Dien Bien Phu, Part II:

The initial stages of the French operation at Dien Bien Phu (DBP) went very well. A parachute battalion seized control of the valley area as planned. An engineer unit then expanded, enlarged, and lengthened the airstrip, to make it usable for a large scale airlift. Slightly greater than a divisions worth of French troops were then airlifted into the DBP area. Battalion strong points were then constructed, in anticipation of a Viet Minh offensive. Artillery was brought in, as well as light tanks. All this effort caught the Viet Minh by surprise. The French did make some calculations with their plan that were appreciated to be risky from the start. Risks, but manageable and acceptable risks. One was that an isolated garrison without a road connecting it to the outside could still be adequately supplied by airlift and airdrop. The French deliberately chose a location for their strategic defensive battle that had no chance for overland resupply or reinforcement. Such a garrison was felt to present such a tempting target for the Viet Minh that they could not resist launching an offensive, and offensive that would fail due to superior French firepower. These French calculations were later to prove incorrect and fatal for the French.

coolbert.