So... This Happened

[nintendo048]

japan is an isolated island nation what else could possible explain it and yeah the chinese and japanese hate each other. Japan has only recently become open to the rest of the world

Only recently? No, they were open to the rest of the world before that. How else would have they received the heavy dose of sinification that they did?
They use the sinic alphabet, zodiac, have adopted various Chinese customs, and even had their language significantly altered by Chinese. They might not have intermarried, though no people are that isolated.

But I agree with you on the subject of intermarriages. I personally believe that intermarriages do not add much to a nation's future, as they certainly promote the same behavior in the offspring.
This is how many of my people's tribes, like the Apar and Bolgar disappeared.
A person that cares about his people, will do his or her best to marry amongst his or her own.

ok fine but as far as isolation go the japanese are the best model for this example besides Genghis khan and the dutch who did they have contact with. and the people they did have contact with were treated poorly and as foreigners and immediately exiled