Japanesse history

How we can trace our grandfather history about japanesse blood we dont have any documents even birthcertificate but my mother told ask that her father is a japanesse blood and also my grand father keep on changing real surname because during world war many american soldier they arrested who has japanesse name thats why they have many names

Did your grandfather live in the U.S.? If so, try the following websites. I found traces of my great grandfather through them. He lived in the U.S. for the final ten years of his life.
https://www.ancestry.com/
https://familysearch.org/

There are also other similar websites. Scroll down and see the list where it says "See also".
https://en.wikipedia.org/wiki/FamilySearch