Final answer:
The French were ousted from Vietnam after WWII due to the military defeat at the Battle of Dien Bien Phu in 1954, which led to the Geneva Conference and the subsequent independence of Vietnam, Laos, and Cambodia.
Step-by-step explanation:
The question pertains to the historical context in which the French were ousted from Vietnam after World War II. The French colonial rule in Vietnam ended after a prolonged conflict known as the First Indochina War. The decisive event that led to the French withdrawal was the Battle of Dien Bien Phu, which took place in 1954. After a 55-day siege, the Vietnamese forces led by General Vo Nguyen Giap succeeded in overcoming the French garrison. The defeat at Dien Bien Phu resulted in the Geneva Conference, where the French agreed to withdraw their troops from Indochina, leading to the independence of Vietnam, Laos, and Cambodia.