Reagan learns that the Union army are holding several Apaches including the infamous Geronimo prisoners and intend to use these as a bargaining chip to get the Apaches to fight the Johnny Rebs alongside the Union Army. However the story turns and the two warring brothers are forced to fight side by side against an Indian attack.
The Last Outpost is pretty much a standard western adventure, of the kind the BBC would show on a Saturday night when I was growing up, but saying that it is a good one and the climactic battle is rousing stuff. It may seem a little simplistic by the standards of later westerns but it was filmed with the sole intention of providing an exciting adventure and it certainly does that.
I'm not sure why the film should be in the public domain but given that it is available to be viewed fully on You Tube then it must be.
I have embedded the trailer below and the full movie can be found by searching on You Tube.