Japan was able to occupy areas in China, Korea, Thailand, and the islands that make up the Indonesia. These islands were controlled at that time by the Netherlands until Japan invaded. The United States and Great Britain were also threatened because Japan was attacking islands in the South Pacific like the Midway Islands and Hawaiian Islands that belonged to these nations. These world superpowers were not going to relinquish their holdings to the Japanese.
I hope this helps! :)
~ erudite