Legitimate authority is a widely touted yet rarely analyzed concept in discourse about war. In this essay, I articulate and analyze the schema of just war theory that has dominated philosophical discourse regarding war since the early medieval period. Although the requirements for a “just war” appear to exceed the simple proclamation by a legitimate authority, in fact, all of the other requirements are subject to the interpretation of the legitimate authority. In other words, just war theory reduces, in actual practice, to the requirement of legitimate authority. A consideration of the nature of contemporary warfare further suggests that just war theory is the vestigial idiom of a world that no longer exists. What remains today of just war theory is a dangerous rhetorical weapon, deployed by the leaders of both sides in every belligerent conflict.