Japan raping China during WWII is what I've seen the most, but I remember a fic where he raped pretty much every single nation that was under the thumb of the Japanese Empire until WWII. (OC's included, I think) That was, uh, ew.

I also remember some fics with China raping Taiwan and Hong Kong, but these were really old and most likely lost forever. And were also all kinds of ew.