In the 19th century, Japan, unlike China, responded to Western pressure to open up to trade not by fighting back but by transforming itself so that, while still geographically in Asia, it became in effect a European country.
Japan decided to hop on the imperialist bandwagon and to become imperialist in every sense of the word.
It acquired the imperialist taste to subjugate other peoples so as to acquire colonies. In this, it outdid its Western mentors, quickly turning Korea, which had recognized China as its suzerain, into a colony and then turned its eyes on the main prize — China itself — and all of Southeast Asia.