The Sworn Sword by George R. R. Martin

A roaming hedge knight and his clever young squire take service with a bitter local lord caught up in a bitter feud, and are drawn into a tense clash of honor, grudges, and shifting loyalties; as political maneuvering and brute force escalate, the knight's code and the squire's secret force difficult choices that reveal the costs of chivalry and the human consequences of small, brutal wars.