The use of shields in warfare as existed at least since the bronze age, and it became a core element of warfare in all european and asian nations. Besides Japan, I don't know any country or place in the world that had active relations with countries possessing shields and just didn't catch on to it. Maybe I just don't know enough about japanese history but it seems like they've never used shields. Why would they not use them if they had knowledge of its existence? Maybe they focused more on armor, therefore making a shield not necessary? Or is there another reason? Maybe they had shields but I've just never seen them before?