I read that the First Indochina War cost France over 100,000 casualties, of which 75,000 were killed. Isn't this an immense loss of life to sustain for the sake of preserving one colony? What motivated France to do it?
Natural resources and prestige.
France wanted the metals (copper, zinc) found in SE Asia, plus natural rubber was still a well-used product, and the French car and tire industry (Michelin, anyone?)
In addition, after its collapse and capitulation in WWII, it's collaboration by its Vichy government, France was at a low point in national morale. The restoration of its colonies taken by the Japanese in conjunction with being named to the UN Permanent Security Council, as if they were actually a world power on the lines of postwar USA, USSR, China or Britain, helped greatly to bring about that healing.