Answer:
Step-by-step explanation:
The problem is they don't. One day you will take a history class that talks about Hiroshima or the Holocaust. They were both tragedies of a kind that is almost impossible to record with no bias.
But what would happen if you read the history from another point of view. Suppose, which I don't think has been done in any school in North America, you were to read about Hiroshima from the point of view of the Japanese. What have they said about it? What will they teach their children? What is the folklore about it from their point of view? Undoubtedly their best historians will record it without bias, but will be the same as what we read? I'm not entirely sure.
That does not answer your question, but I have grave doubts that it is possible. Personal bias always comes into everything. I will say this about your question: we must do our best to present the facts in an unbiased manner. That's important because we need to have a true picture of what happened. Many times it is because historians don't want humanity committing the same errors as the events they are trying to make sense of.
So far we have not dropped an atomic weapon on anyone else. But there have been holocausts after the European one. What have we learned? That six million is a number beyond our understanding, and we have not grasped the enormity of the crime, bias or no bias.