If You Liked Dances With Wolves or Avatar, You Might Like This.
Buckskin Brigades by L. Ron Hubbard
Way back before L. Ron Hubbard created his self-help dynasty called Scientology, he was a writer of pulp fiction. He penned a number of science fiction titles, some of which are still popular today, such as Battlefield Earth. But being the intrepid soul that he was, he also tried his hand at writing a few westerns. I just finished lisitening to Buckskin Brigades on CD, read by Bruce Boxleitner. (I think Mr. Boxleitner could read the yellow pages and make it exciting.)
Basically this is a romance, in the same vein as Dances with Wolves or even the more recent Avatar by James Cameron. A man from an invading culture joins with the indigenous peoples (in this case the Blackfeet, of which Hubbard claimed to be a blood brother - but that's another story) to fight against the overwhelming forces of his own people. And like the two previous titles, at the end of the story the tide as been turned, at least for awhile. Oh, and did I mention he falls in love with a woman in the indigenous tribe?
So, if you want realism, keep looking, but if you like big bad bullies to get their comeuppance one in awhile this is a title worth checking out.