I've always been interested in the history of the West, our country and particularly as it relates to the Native Americans - the original Americans.
Second, this epic tale allows the audience to actually listen to the Native Americans and receive their wisdom. Spielberg conveys the respect for Native Americans that is normally lacking in Western films.
When Spielberg was talking with the other executives about making this movie, he said he was only interested if they were going to tell the truth about the westward crossing, which means how the Native Americans were disrespected.