The 1992 film “Điện Biên Phủ” is based on a true historical event, the Battle of Điện Biên Phủ (March–May 1954) in northwestern Vietnam.
That battle was the decisive confrontation between the French Union forces and the Việt Minh (led by General Võ Nguyên Giáp). It ended in a catastrophic French defeat, forcing France to abandon its colonial rule in Indochina and directly leading to the Geneva Accords, which divided Vietnam into North and South.
The film portrays real aspects of the battle — such as the French paratroopers, the artillery duels, and the desperate defense of the valley — but it also mixes in fictionalized characters and subplots (for example, a love story between a French journalist and a Vietnamese woman) to dramatize the events.
So, in short:
True story: The battle itself, the outcome, and many of the military events.
Fictional elements: The personal stories and dialogues, which serve to humanize and dramatize history.