To The Last Man by Zane Grey

An ancient feud between two frontier families is inflamed when one of the families takes up cattle rustling. In the grip of their relentless code of loyalty, they fight a war in Tonto Basin, desperately, doggedly, neither side seeing the futility of the conflict. Surrounded by this volatile environment, young Jean finds himself hopelessly in love with a girl from whom he is separated by an impassable barrier.