Recent just war thought has tended to prioritize just cause among the moral criteria to be satisfied for resort to armed force, reducing the requirement of sovereign authority to a secondary, supporting role: such authority is to act in response to the establishment of just cause. By contrast, Aquinas and Luther, two benchmark figures in the development of Christian thought on just war, unambiguously gave priority to the requirement of sovereign authority as instituted by God to carry out the responsibilities of ensuring a just and peaceful order in the world. On this conception it is the sovereign, in deciding whether to resort to armed force, who must make sure to satisfy the other moral requirements of the jus ad bellum. This paper examines Aquinas and Luther on sovereign authority for use of armed force. Recapturing the importance of this conception is important both for the proper understanding of just war tradition and for working out its implications for such contemporary issues as humanitarian intervention and "regime change."