Finn’s War by John Legg
When the fur trade ends, Finn Hazelwood finds himself restless and rootless. As the years pass, his life takes him into some strange things — horse “gathering” in California with some former mountain men, farming in Oregon Country, marshaling in Nebraska. Finally he lands in the gold country of what becomes Montana Territory, looking for his old partner from the mountain days. There he is recruited to rescue female captives from his old enemy — the Blackfeet. And he finds himself in a war between the Indians and vigilantes.