Residents inspect damage from what activists said was an airstrike by forces loyal to Syrian President Bashar al-Assad on Douma, eastern Ghouta, near Damascus in October. (Bassam Khabieh/Reuters)

Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week we’re talking about just war theory. Need a primer? Catch up here.

Nolen Gertz is Assistant Professor of Applied Philosophy at the University of Twente in the Netherlands and Inamori Research Associate in Military Ethics at Case Western Reserve University. He is the author of The Philosophy of War and Exile.

Shortly after British lawmakers voted to authorize military airstrikes in Syria on Wednesday, philosopher A.C. Grayling tweeted, “War is an evil only ever justified by engaging in it to abate a greater evil.” This sentiment belongs to the tradition known as just war, the intellectual tradition of attempting to determine how best to — as Grayling makes clear — weigh evils.

The language of “evil” here is important. Though the tradition of just war has antecedents that go back at least as far as Ancient Greece, its most systematic form comes out of Catholic scholarship, in particular the work of Saint Augustine and Saint Thomas Aquinas. As the Catholic Church was merging with the Roman Empire, the problem of merging the philosophies of Jesus Christ and of Julius Caesar became harder and harder to avoid. Augustine’s answer to the problem of how a Christian soldier could “turn the other cheek” was to invoke the idea that this turn need not be of one’s literal cheek, but rather of one’s metaphorical cheek. In other words, a Catholic could follow Caesar by fighting for the Empire so long as he followed Christ by not wanting to fight. A Catholic soldier must never fight with a “lust for war.

[Other perspectives: The Islamic State’s members believe they are fighting a new Crusade. They’re wrong.]

While the concepts found in the tradition of just war have persisted to this day, the vital theological underpinning of the tradition has not. Whereas Christian soldiers could, in the time of Augustine and Aquinas, turn to their priests for guidance on how to make their now “dirty hands” clean again through acts of penance that were carefully calculated by the Church, today there is no such guidance and no such penance. Secularized just war theory instead attempts to replace the certainty of God and the calculations of the Church with the uncertainty of “common morality” and the calculations of consequentialism.

These seemingly theoretical issues with just war theory are, in fact, very practical and very widespread, particularly for the combatants and veterans who are forced to live with the consequences of our “justified” wars. War nerves, soldier’s heart, shell shock, post-traumatic stress disorder and traumatic brain injury are just some of the names we’ve given to the suffering of soldiers who have faced the very real dilemma of perpetrating the immoral in the name of morality.

Whether we intervene in Syria with bombs from the sky or “boots on the ground” it is necessary to remember that intervention requires that people fly those bombs, that people wear those boots. The rhetorical device of focusing on the means of war rather than the people tasked with war-fighting is essential both to how we “justify” war and to how we help to contribute to the suffering of war-fighters.

The Islamic State must be stopped — that is certain — but the most “just” way to achieve this goal is not certain. Bombing and invading Syria will certainly kill Islamic State fighters, but it will also kill civilians and help the group to recruit new fighters. And so long as Syrian President Bashar al-Assad is in power, we cannot ignore that to fight the Islamic State is to help Assad, which is yet another way to help the group recruit new fighters.

As John F. Kennedy argued in 1961, echoing Pericles and John Winthrop, the role of the United States is to be a “city on a hill.”  If our goal is to defeat the Islamic State, to win the “war on terror,” then we must not fight terror with terror, but rather prove that the United States is not the Islamophobic, war-hungry superpower that the terror group requires us to be.

As the philosopher Immanuel Kant warned, invoking the language of morality to do the immoral is to do damage to morality. To enlarge the sphere of what is just and what is justified to include the killing of civilians and the suffering of combatants is again to only help members of the Islamic State feel that their use of similar means — death and destruction — for what they see to be similar ends — the defeat of “evil” — can also be just and justified.

Explore these other perspectives:

Jeff McMahan: Syria is a modern-day holocaust. We must act.

Marc Pierini: For Europe, war against the Islamic State is justified. But is it worth it?

Thomas Donnelly: The fight against the Islamic State is no ordinary war

Neta C. Crawford: Is the Islamic State a government or a criminal gang? The answer will determine how we fight.

Aaron MacLean: It’s time to end the Islamic State

Thomas Madden: The Islamic State’s members believe they are fighting a new Crusade. They’re wrong.

Andrew Tabler: Sometimes the enemy of your enemy is still the enemy