Political map of South & Southwest Asia 17 January 1941 (Japanese Intervention in French Indochina): Germany's defeat of France in 1940 left the French overseas empire vulnerable. Japan, which was already at war with China, decided to exploit this opportunity and invaded northern French Indochina (Japanese invasion of French Indochina). It was soon followed by Thailand, which retook territories French Indochina had annexed earlier in the century.