From Wikipedia

The 10,000 Day War
Episode Two

 Dien Bien Phu

The Battle of Dien Bien Phu (French: Bataille de Diên Biên Phu; Vietnamese: Chiến dịch Điện Biên Phủ) was the climactic confrontation of the First Indochina War between the French Union's French Far East Expeditionary Corps and Viet Minh communist revolutionaries. The battle occurred between March and May 1954 and culminated in a comprehensive French defeat that influenced negotiations over the future of Indochina at Geneva. Military historian Martin Windrow wrote that Điện Biên Phủ was "the first time that a non-European colonial independence movement had evolved through all the stages from guerrilla bands to a conventionally organized and equipped army able to defeat a modern Western occupier in pitched battle."

As a result of blunders in the French decision-making process, the French began an operation to create an air-supplied base at Điện Biên Phủ, deep in the hills of northwestern Vietnam. Its purpose was to cut off Viet Minh supply lines into the neighboring Kingdom of Laos, a French ally, and draw the Viet Minh into a battle that would cripple them. Instead, the Viet Minh, under Senior General Võ Nguyên Giáp, surrounded and besieged the French, who were unaware of the Viet Minh's possession of heavy artillery (including anti-aircraft guns) and their ability to move such weapons to the mountain crests overlooking the French encampment. The Viet Minh occupied the highlands around Điện Biên Phủ and were able to fire down accurately onto French positions. Tenacious fighting on the ground ensued, reminiscent of the trench warfare of World War I. The French repeatedly repulsed Viet Minh assaults on their positions. Supplies and reinforcements were delivered by air, though as the French positions were overrun and the anti-aircraft fire took its toll, fewer and fewer of those supplies reached them. After a two-month siege, the garrison was overrun and most French forces surrendered, only a few successfully escaping to Laos.

Shortly after the battle, the war ended with the 1954 Geneva Accords, under which France agreed to withdraw from its former Indochinese colonies. The accords partitioned Vietnam in two; fighting later broke out among rival Vietnamese forces in 1959 resulting the Vietnam (Second Indochina) War.