白人の日本 is meaning " Japan of white" so that word is weird. に is ~ for,to, よる(因る)is meaning such like a 〈原因となる〉 be due [owing] to; be caused 《by》or 〈応じる, 基づく〉 be based [founded] on; depend [rely, hang] on.
Therefore I should translate " We never forgive about contempt to Japanese by white"
In this case, "by" is will be "による”.
" 白人による日本侮蔑は許さないぞ!<はくじんによるにほんぶべつはゆるさないぞ!>
による is very difficult expression so you just remember as a example for that word.