Dien Bien Phu is “one of Vietnam's most important battlegrounds,” explains Rusty Compass, and it's also “one of its best preserved.” This is the site of the 1954 French military disaster that “established once and for all the military credentials of Ho Chi Minh's revolutionary forces.” The town sits in the Muong Thanh Valley, “surrounded by heavily forested hills,” writes Lonely Planet. “The scenery along the way here is stunning, with approach roads scything through thick forests and steep terrain.”
Hill A1 is an “entrenched fortification” considered a leading French fortress, explains Maze Vietnam. “The remnant of tanks is displayed on the hill in remembrance of the fascinating history,” and look for the hole atop Hill A1 “which is a result of a massive explosion in Dien Bien Phu battle.” There’s nothing at this site written in English, so “please make sure that you have read about the Dien Bien Phu victory to have an insight” into its history, advises Maze Vietnam. Or as Travel & Leisure puts it: “The experience of a physical place often depends as much on the creative powers of the visitor as it does on a walk around the area … indeed it is almost mandatory to arrive at Dien Bien Phu with a vision of what happened, and why it matters, already in mind.”
So what happened and why? Here’s how Wikipedia explains the key facts: The Battle of Dien Bien Phu was the climactic confrontation of the First Indochina War between the French Union's French Far East Expeditionary Corps and Viet Minh communist-nationalist revolutionaries. It was, from the French view before the event, a set piece battle to draw out the Vietnamese and destroy them with superior firepower.
The battle occurred between March and May 1954 and culminated in a comprehensive French defeat that influenced negotiations underway at Geneva among several nations over the future of Indochina.
As a result of blunders in French decision-making, the French began an operation to insert, then support, the soldiers 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 tactically draw the Viet Minh into a major confrontation in order to cripple them. The plan was to resupply the French position by air, and was based on the belief that the Viet Minh had no anti-aircraft capability. The Viet Minh, however, under General Võ Nguyên Giáp, surrounded and besieged the French. The Viet Minh brought in vast amounts of Soviet made heavy artillery (including antiaircraft guns).
They moved these weapons through difficult terrain up the rear slopes of the mountains surrounding the French positions, dug tunnels through the mountain, and placed the artillery pieces overlooking the French encampment. This positioning of the artillery made it nearly impervious to French counter-battery fire.
The Viet Minh opened fire with a massive artillery bombardment in March. After several days the French artillery commander, Charles Piroth, unable to respond with any effective counterbattery fire, committed suicide.
The Viet Minh occupied the highlands around Điện Biên Phủ and bombarded the French positions. The French repeatedly repulsed Viet Minh assaults on their positions. Supplies and reinforcements were delivered by air, though as the key French positions were overrun, the French perimeter contracted and the air resupply on which the French had placed their hopes became impossible. The garrison was overrun in May after a two-month siege, and most of the French forces surrendered. A few of them escaped to Laos. The French government in Paris then resigned, and the new Prime Minister, the left-of-centre Pierre Mendès France, supported French withdrawal from Indochina.