"I simply could not comprehend the incredible gamble
that Generals Navarre and Cogny were accepting that spring of 1954."
- - Bob Seals.
Here with some quick and dirty calculations, the area, the terrain as defended by the French in square kilometers first at Na San and then Dien Bien Phu [DBP].
In both instances the French content to fight strictly a defensive battle, battalion sized strong points manned against a concerted Viet Minh onslaught.
In the case of Na San, the French ABLE TO HOLD. In the case of Dien Bien Phu, the French NOT ABLE TO HOLD.
1. Na San.
Total area defended equals about thirty-one  square kilometers.
The center position surrounding the airfield at Na San defended by a series of strong points equal to about five  square kilometers.
An outer ring of defensive positions with an inner right centered on the airfield.
Troop strength for the French at Na San about: 15,000.
Troop strength for the Viet Minh at Na San about: 60,000 [?].
2. Dien Bien Phu.
NOT counting strong point [battalion size] Isabelle, the French attempted to defend about eighteen  square kilometers.
This includes the area covered by eight of the nine strong points at DBP, those battalion strong points being: Gabrielle, Anne-Marie Beatrice, Huguette, Francoise, Claudine, Eliane, Dominique.
The inner defenses of DBP centered around the airfield equal to about eight  square kilometers.
Isabelle located about 4500 meters to the south of the main landing strip at DBP, Isabelle severely isolated in an almost unconscionable manner.
[Isabelle was protecting an "auxiliary landing strip".]
Troops strength for the French at the start of the battle about 13,000 [10,000?] men.
Viet Minh troop strength at the start of the battle about 48,000 fighting soldiers, not counting logistical personnel.
Three of the eight battalion sized strong points as manned by the French Union troops falling on the first three days of the battle, the defenders beaten badly, units almost annihilated. Giap using concentrated focused energy obliterating his opposition almost in a leisurely [?] manner!
" [the first day of the battle] . . . A few minutes after 1700 hours,
more than 200 artillery pieces opened fire, in a whirlwind
of blast and shrapnel which ripped through the French positions . . .
The guns stopped as the sun fell, which was the signal for the
first massed infantry attack. The Viet Minh 312 Division with over
7,000 fighting men swarmed on to Beatrice, wiping out the battalion
of 700 men stationed there. Only 200 escaped . . . The next three days
followed a similar pattern - - a hurricane bombardment followed by
an overwhelming assault on one of the strong points."
What was possible at Na San was not possible at Dien Bien Phu?
And from the wiki entry we find the three reasons for French defeat at DBP being:
1. French occupying low ground, Viet Minh occupying high ground.
2. Meticulous planning and preparation of the Viet Minh. No hurried and rushed action.
3. French re-supply severed, becoming impossible.
The French plan at DBP was worthy, but the execution was poor? With disastrous consequences. The French high command did gamble. The thought was, wager a lot, but gain a lot if you win. Reasoning not totally without merit.
Navarre rolled the dice and it came up "snake eyes"!