Question

What was the Japanese expansion during WW2?

Answer 1

Slightly before the Pearl Harbor attacks but because that took place across the International date line are listed on the 8th of December Japan began attacks though out Asia.

Explanation:

image credit http://go.grolier.com/atlas?id=mh00084

This shows Japanese expansion from 1875. The yellow area are that initiated simultaneous with the Pearl Harbor attacks and achieved by the end of 1942.

The British, the French and the Dutch were all weakened by the War with Germany. The French were occupied but Vichy France remained. The Japanese were able to take advantage of this by occupying French Indochina in 1940 but left the French administration in place. They had valuable air and naval bases to stage from.

The Japanese also intimidated the Taiwanese into letting them cross Thai territory to attack the Malay Peninsula. Thailand was not occupied.

The areas were conquered in a few months, surprisingly quickly and efficiently.
https://worldview.stratfor.com/article/japans-territorial-expansion-1931-1942