The history books are written by humans who have definite prejudices. When a certain race is in "power/vogue", then all of a sudden, the history books take on a different flavor.
Haven't you known families that colored their own history when writing their genealogy? It's true of historians... it's all in what the flavor of the current administration is when the books are written.
I used to believe that history books were true.... They aren't. The dates are true, the spin is not.