if you got any china or jappan history question . i think i can offer some help .

i think you should choose a period , that china is conquerd by one faction . like Westen Han B.C 206─A.D 9 . at that time , in my memory , just have no more than 6 factions beside china . inculding huns and other west factions and south barbarians.
and the jappan send their first ambassador　in 57 BC, and become a feudatory of china(i don't know how to describe in english , i think it must be said china accepted a country was far from mainland china , and accepted a barbarian faction submit to him .) .and japan began to get many china's civilizations from then on . well , i think japanese was so friendly as a barbarian than now :(