While I have no idea how difficult it is - probably -very- difficult - it is not impossible. My father for some reason knows an American who is a tenured professor of Japanese History in a Japanese university. (Or at any rate, did know such a person when he was a professor some twenty years ago, I'm not sure if they are still corresponding or what has happened to the man's career. Apparently, when telling Japanese people of this acquaintance, they were universally disbelieving, which gives you some idea of the rarity of the feat. I doubt it's -quite- that rare now, but I don't think its gotten too much easier either. )
Anyway, asking here - or on any language learning forum - is not likely to help. You need to be checking directly with schools that offer graduate studies in Japanese History in Japan. Of course, to do that, you need to be able to ask in Japanese. You'll also need to be able to take graduate level classes in Japanese.
If that doesn't worry you, then go for it. You'll never know if you don't try. If you get accepted into a program, I think you'll likely find your way somehow - even if you don't gain a career in Japan you'll have impeccable credentials for teaching in the west. On the other hand, if even searching and applying sounds impossible to you, then I suggest you consider studying and teaching history at home.
