The correct answer is - China.
Japan had an expansionist political view prior and during the World War II. The country lacks natural resources, so its leaders decided that it would be wise if they manage to conquer new territories that are abundant in natural resources.
Initially, Japan had set its goals on Manchuria, China, Korea, and parts of Southeast Asia. Later its goals were to conquer the Pacific islands, and in the later stages it even set its goals on Australia and the west coast of USA.
The Japanese managed to conquer Manchuria, the part of China they saw as most useful. They actually managed to did it very quickly and very efficiently, and the reason for that was the good organization of the military, as well as the more advanced weaponry.
Australia and USA's west coast though were never invaded, and they only remained as a goal but nothing more for the Japanese leaders.