In a campaign full of accusations and differing viewpoints, Mitt Romney and Barack Obama spent their third presidential debate agreeing with each other on foreign policy. Indeed, each was heard saying things more typically associated with the other’s party.

Romney described the multi-faceted approach to combating Islamist terrorism, noting that “we can’t kill our way out of this mess.” Obama declared that Muammar Gaddafi had more American blood on his hands than anyone except Osama bin Laden, and justified his intervention in Libya on the grounds that “America has to stand with democracy.” Both candidates espoused a kind of pragmatic American exceptionalism in international affairs: Romney argued that the mantle of leadership for promoting principles of peace has fallen on America’s shoulders, and Obama called America “the one indispensable nation.”

They gave less thoughtful consideration, though, to the ethical issues at stake in foreign policy. Both presumed that preemptive war and the invasion of a foreign country are justified, as long as they aren’t too costly and serve the interest of America’s national security. They passed briefly over the growing number of unmanned drone strikes against terror suspects, failing to mention how many civilian casualties the strikes have cost.

Our public conversation about the use of American military power needs deeper answers to complicated questions. We need wisdom from cultures and time periods not our own, and guidance from a debate about the right use of lethal force that spans many centuries. In seeking both of these things, the just war tradition can guide us.

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For this we need look no farther than David D. Corey and J. Daryl Charles’s The Just War Tradition: An Introduction. It’s easy to think that just war theory is a shell game that allows leaders to justify any action they favor, or a set of boxes that nations must check before attacking their enemies. Corey and Charles, however, challenge those conceptions. They see just war not as a convenient, compact theory, but as a conversation that has unfolded over time—a tradition, like those associated with Plato, Aristotle, or Aquinas, in which different voices bring new questions, distinctions, and conclusions to the debate about whether, when, and how war is morally permissible.

To help us understand what the just war tradition is, Corey and Charles first distinguish it from two philosophies it isn’t: realism and pacifism. Realism justifies war on the grounds that moral rules for our personal and domestic affairs don’t apply to international relations. Pacifism, to the contrary, holds that war is always wrong, whether because of pragmatic reasons (its costs outweigh its gains), deontological reasons (it violates universal principles of right), or religious reasons (it runs contrary to the teachings of Jesus).

As attractive as realism and pacifism may be, Corey and Charles argue that “they leave their adherents with virtually no language, no conceptual tools, no guiding principles for evaluating the justice or injustice of particular wars or particular practices of war.” The just war tradition, by contrast, is a framework that “offers a rich, highly inflected language, a storehouse of categories, concepts, and commonplaces developed over centuries of reflection, in which the moral particulars of war can be examined.”

By using the same principles of justice that apply to all moral affairs, the tradition aims “to consider how wars might be waged justly” when we decide whether to go to war (ius ad bellum), how to conduct a war (ius in bello), and what to do when the war is won (ius post bellum).

For Corey and Charles, reflection on just war properly begins with Augustine. Augustine read Scripture allegorically to uncover principles for just interaction during conflict. For example, in considering Jesus’ exhortation to “turn the other cheek,” he notes that Christ responded to being struck with a rebuke (John 18:23) and that St. Paul, when he was struck at a trial, replied, “God shall strike you, you whitewashed wall!” Faced with an apparent contradiction, Augustine concludes that “turning the cheek” is not a matter of literal action, but of ensuring that patience is not overcome by the desire for vengeance.

Augustine’s thought on war also offers the concept of benevolent harshness (benigna asperitas). Just as a parent lovingly chastises a child not to cause pain, but to teach him virtue and uphold justice, so nations can wage war with the welfare of victims and the conquered in mind. Still, Augustine repeatedly calls war a “necessary evil.” It is evil because sin gave rise to it, and, indeed, is likely to lead many to further sin. But it can be necessary out of concern for those afflicted by injustice, or for the unjust themselves.

The first condition for just war is a just cause. For Augustine, physical death and pain are not the greatest evils; sin is. Therefore, just wars must be acts of moral necessity brought about by the desire to correct a wrong. Just wars are reactive, not proactive, and they must be prudent.

The second condition for just war is that a proper authority must wage it. Private citizens cannot conduct wars.

The third condition is a goal of peace. To Boniface, the Roman governor of Africa, Augustine wrote: “The will should be concerned with peace and necessity with war, so that God might liberate us from necessity and preserve us in peace. Peace is not sought in order to provoke war, but war is waged in order to attain peace.” Finally, at the war’s conclusion, mercy should be shown to those who have been conquered or captured, that they might know peace as well as the victors.

Augustine’s guidelines for just war gave the authors that followed him a theological framework to develop. In his consideration of sins against charity in the Summa Theologiae Thomas Aquinas asks whether some kind of war is licit. He concludes that sovereign authority, a just cause in response to wrongdoing, and the right intention of charity on the part of the belligerents are necessary for a war not to be sinful.

Unlike Augustine, however, Aquinas argues that there are limits to the obedience that a subject or soldier owes to his leader: The leader’s authority must be legitimate, his command must be for the common good, and its burdens must be equal and proportionate. With these considerations, Aquinas opened the door for forms of civil disobedience and conscientious objection for those involved in war.

During the Reformation, the authors argue, Martin Luther abandoned many traditional distinctions, focusing instead on obedience to the sovereign as a means of loving one’s neighbor. For Luther, only an actual attack from an enemy can prompt a Christian reluctantly to take up arms out of defense of his neighbor. Once in war, however, the soldier was given free rein to “kill enemies without scruple”; Luther’s thought on love of enemy does not mix with his thought on war. However, Luther also made an important advance in the just war tradition by condemning all forms of holy war.

John Calvin, by contrast, advocated a Christian polity that could commit religious violence to guard the polity’s faith, but could not meddle in the faith of foreign countries. Siding with the previous tradition, he also rejected Luther’s limitation of just cause to self-defense, but he more strictly argued that “everything else ought to be tried first before the recourse to arms.”

As the early modern period progressed, thinkers began to examine the question of just wars through criteria based more on natural law, and less on theological principles. They also further developed considerations of ius in bello. Writing in response to the Spanish and Portuguese conquest of the New World, Francisco de Vitoria argued that war could only be waged in response to a wrong incurred. The Native Americans were persons just as Europeans were, and they could not be Christianized by the sword.

Moreover, war should not be the act of one prince alone, but the opinion of many wise men. War can only be waged after guilt and lawlessness have been established, and must be waged “for the good of the whole world.” Likewise, Vitoria’s contemporary Francisco Suarez argued that natural law forbade the intentional killing of unarmed innocents and emphasized the moral and political unity of the human race as a whole. In so arguing on the basis of universal principles,Vitoria and Suarez are considered founders of international law.

John Locke adopted and modified the just war tradition by arguing on the basis of natural rights, in light of his own view of the state of nature. For Locke, the relations between states resemble the relations between persons in the state of nature. Since Locke does not offer a developed theory of just war, Corey and Charles therefore extrapolate what it might be from his relevant writings.

In their view, Locke would argue that a just war must respond to aggression, but such aggression need not have borne fruit in an actual attack. Any serious threat of aggression can result in military action, including preemptive or preventive action. But that action must run parallel to the natural ends of punishment: restraint of the aggressor, hope of reforming its conduct in the future, reparation for damages, and deterrence of other states from imitating its aggression. More explicitly, Locke argues that conquerors are allowed to kill or enslave enemy combatants as violators of the laws of nature, but they cannot harm or seize the property of noncombatants and (with some exceptions) combatants. Finally, the conqueror has no right of government over the conquered; if the latter’s former government has been destroyed, they “are at liberty to begin and erect another to themselves.”

Corey and Charles briefly consider Kant as a voice outside of, but in dialogue with, the just war tradition before turning to contemporary writers, including Michael Walzer, William O’Brien, Jean Bethke Elshtain, James Turner Johnson, and John Courtney Murray. To recap just a couple of their ideas: In a manner redolent of Augustine, Johnson argues that just war thinking begins not with preserving peace and a presumption against war, but with preserving justice and a presumption against injustice, which is a requirement of rightly construed love. And Murray writes against the persistent fallacy that war and peace are “two discontinuous and incommensurable worlds of existence and universes of discourse, each with its own autonomous set of rules, ‘peace’ being the world of ‘morality’ and ‘war’ being the world of ‘evil.’”

It is clear that the pacifism espoused by some Americans embraces such a dichotomy: War is evil, therefore it is not an option. But the apparent moral realism of our ruling elite—or at least of our two presidential candidates—can veer toward this dichotomy, too.

For both the realist and the pacifist, the ethics of love offer nothing to say to those in war. For the just war tradition, though, war is a fact of sin in this world and, though harsh and fraught with peril, it can be a means of living out the love and justice of God in the world. This tradition of reflection must inform our thinking on foreign policy today as well. Platitudes, checklists, and positive laws are not enough in an age of a nuclear Middle East, drone strikes, and preemptive interventions.