Which of course is also the go-to "excuse" of the modern Japanese right-wingers to deflect any blame from the "pure" Japanese for what they did.
I’ll repeat it: the primary reasons which caused Japan to become “imperialist” are directly tied to Western imperialism. The Japanese emperor was mostly a political symbolic figurehead that united the Japanese islands, not a Roman emperor or British monarch intent on conquering the world.
This is all basic Japanese history.
Wikipedia article on Shogun[1] has a nice quick summary on pre-modern Shogunate system. It’s similar to British constitutional monarchy in a sense: the emperor was to delegate all power to the government and was to remain within instructions and supervisions of the government.