I know nothing about native american history, but I had a long discussion with a professor from Houston (Tx) University while he was visiting us a couple of years ago, and he told me that the movie was absolutely BS and pure tree-hugger fantasy (hey I love trees too but you know what I mean...)
Then I read a novella by Dan Simmons, certainly one of the living writers I respect most, in which the movie is mentioned.... Basically D. Simmons thought the same.
Honestly I enjoyed the movie the 1st time I saw it. At the light of what I've since learned, I have no desire at all to see it again.