Gratian (Decretum Gratiani)

The Virtuous Ruler: Hildegard of Bingen and the Question of Political Authority in Just War
The Virtuous Ruler: Hildegard of Bingen and the Question of Authority in Just War

For Hildegard, rulers who blatantly violate the virtuous example set by Christ lose their authority to rule. Following from that, an unvirtuous ruler would necessarily fail to be a right authority that can legitimately wage war.