Earliest Revisionist Western I Know of

Everyone talks about the anti-Westerns of the 60s and especially the 70s (buffalo BIll, Little BIg Man, etc), and then of course Dances with Wolves in the 80s, but this is a film I can't believe I've never seen before, and yet, I now know why.

It is SO revisionist. To have one of the heroic lead characters call the US Union army uniform a 'dirty blue rag' in 1955? A film about three fur trappers who get embroiled in the machinations of a US Army outpost facing A Native American tribe with a madman colonel who thirsts for a bloody unnecessary battle for no more reason than his own twisted ego?

I'm impressed by the subtleties of this film and so glad it breaks the mold of the time, and the self-congratulatoin of later deacedes. All the lead actors are top tier as well. Incredible!