We construct a database linking European royal kinship networks, monarchies, and wars to study the effect of family ties on conflict. To establish causality, we exploit decreases in connection caused by apolitical deaths of rulers' mutual relatives. These deaths are associated with substantial increases in the frequency and duration of war. We provide evidence that these deaths affect conflict only through changing the kinship network. Over our period of interest, the percentage of European monarchs with kinship ties increased threefold. Together, these findings help explain the well-documented decrease in European war frequency.