Why was the North Vietnamese victory over the French at Dien Bien Phu in 1954 an impressive military feat?Aside from the fact they defeated an imperial army, so in terms of strategy and tactics.
In 1954, the French military was not on par with the American military in Korea or the British military elsewhere in the world, as the French economy and military had not yet fully recovered from the devastation and occupation of World War II. So their army was not as equipped in terms of fully modern weapons as one might think. They were especially short in terms of jet aircraft and close air support.
The French had specifically chosen Dien Bien Phu because it was isolated with few usable roads that Giap might use to bring numbers and artillery to bear. By luring him into what they thought was a trap, the French were looking for a decisive victory to end the war.
So the truly impressive part of the Viet Minh's victory under General Vo Nguyen Giap was that he and his army overcame the limitations of the terrain to win anyway. Giap brought 60,000 troops with him through jungle and across rugged mountainous territory, and amazingly, his men brought disassembled artillery pieces with them (!!), put them together on the commanding ridges around Dien Bien Phu and used that fire to destroy the airfield, preventing French resupply and closing the trap.
The resulting siege effectively diminished the French force there, killing 1/7 of the 16,000 French soldiers and wounding a third of them. The last impressive part involved a coordinated attack on the Haiphong airfields that destroyed the limited air force the French had. Urgent requests went out to the Americans in Korea to conduct bombing raids, which were refused by Eisenhower.
The battle also solidly established Giap as a master tactician.