To the Last Man by GREY, Zane

The story follows an ancient feud between two frontier families that is inflamed when one of the families takes up cattle rustling. The ranchers are led by Jean Isbel and, on the other side, Lee Jorth and his band of cattle rustlers. In the grip of a relentless code of loyalty to their own people, they fight the war of the Tonto Basin, desperately, doggedly, to the last man, neither side seeing the futility of it until it is too late. And in this volatile environment, young Jean finds himself hopelessly in love with a girl from whom he is separated by an impassable barrier. (Summary by Wikipedia)