The Union of Indochina united Vietnam, Cambodia, and under French rule.

The statement above is true. The Union of Indochina united three Vietnamese regions, Cambodia, and Laos who were under the French rule. The capital of these three united countries was originally in Saigon but was eventually moved to Saigon then to Da Lat and eventually back again to Hanoi. This was dependent on the move of the French colonies in the Indochina areas.