Final answer:
Vietnam, Cambodia, and Laos were once collectively known as French Indochina, a federation under French colonial rule established in the late 19th century and lasting until the mid-20th century.
Step-by-step explanation:
The three countries that were once known as "French Indochina" are Vietnam, Cambodia, and Laos. This region was under French colonial rule from the late 19th century until the mid-20th century. France established control over these territories after military engagements and political treaties. By 1887, France had created a colonial federation consisting of Vietnam (then divided into three separate regions: Tonkin, Annam, and Cochin China), Cambodia, and later Laos, which was added to French Indochina after conflicts with Siam (now Thailand).
While other powers like Japan briefly took control during World War II, after the war ended, the French attempted to reassert their dominance. This ultimately led to a push for independence. Thus, Vietnam played a central role within French Indochina, leading the region's move toward independence that would result in the dissolution of French rule.