Battle that was effect of French strategy to establish fortified bases close to the routes that Viet Minh forces were using to resupply their units. French used it before on smaller scale near Na San, where base supplied only using air transports was able to withhold their position and cause a lot of casualties to attacking Viet Minh forces.
In Dien Bien Phu, that was on a major supply route towards Laos, French paratroopers managed to establish their position and prepare the airstrip, but due to lack of intelligence reports they were unaware that large number of enemy artillery was relocated on the surrounding hills. Vietnamese forces begun the bombardment of the French soldiers and at the same time the Viet Minh anti-aircraft guns stopped any support for them.
After almost 2 months of fight the French positions were overrun - they were outnumbered (14,000 against almost 50,000 attackers), exhausted and had virtually no chance of being resupplied from outside. 11,000 were captured and forced to take 600 km march to their POW camps.