A Syrian refugee girl stands in a building on June 27, 2015 in Syrian Kurdish city of Amuda, after running away from clashes between regime forces and the Islamic State group. (UYGAR ONDER SIMSEK/AFP/Getty Images)

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.

Jeff McMahan is White’s Professor of Moral Philosophy at the University of Oxford.

With the attacks in Paris and the destruction of the Russian passenger plane, the Islamic State has brought its war to areas beyond the Middle East. This was predictable, as the aim of its war is to expand the newly-declared caliphate until it encompasses the world, exterminating apostates and converting or subjugating infidels in the process. There will, therefore, be more such massacres.

The threat to those in the West, however, is minor compared to what many of the roughly eight million people who live — or die — under the rule of the Islamic State are being forced to endure today. Entire cities, such as Raqqa in Syria and Mosul in Iraq, have been under the Islamic State’s control for more than a year. In areas where they rule, beheadings in public spaces are daily occurrences. Execution by the sword is the penalty for offenses including apostasy, blasphemy, and homosexuality. According to many members of the Islamic State, indications of apostasy among Muslims include shaving, voting, selling alcohol and being Shiite rather than Sunni.

Adultery is punished by stoning, theft by mutilation, consumption of alcohol by 80 lashes, and other offenses by crucifixion. Trials, to the extent that they occur at all, are conducted by fanatical clerics and may last only a few minutes. Hostages are routinely tortured. Captured infidel women, such as those from the ethnic and religious Yazidi minority in Iraq, are sold as sexual slaves whose treatment is governed by a set of arbitrary religious ordinances which state, for example, the conditions in which it is permissible for a man to rape a pre-pubescent girl. Sunni males as young as seven are being conscripted into the fighting units, and are taught techniques of beheading by practicing on captured soldiers from the Syrian army. The least intelligent are chosen for suicide missions.

[Just War Theory: A primer]

A month ago, I visited the Auschwitz and Birkenau concentration camps in Poland. I had read many memoirs by survivors and various accounts by historians, but being physically present in the gas chambers, dungeon cells, and barracks where more than a million people were murdered produced in me a visceral repugnance — a sensation that now recurs when I read about the daily atrocities perpetrated by members of the Islamic State.

We now rightly think it shameful that evidence about the camps was ignored, that efforts to liberate them were not made earlier, and that Jewish refugees were turned away from the US, Britain, and other countries. If camps such as Auschwitz and Birkenau were operating today, most of us would think it inconceivable that it could be morally permissible to stand by and allow them to continue. Yet what occurred in these camps — enslavement, efforts to exterminate certain groups, execution of homosexuals, and so on — is being replicated at this moment in the areas controlled by the Islamic State. These monstrous crimes have been occurring for more than a year while we have stood by and allowed them to continue. It is shameful that the aim of defeating the Islamic State has become an urgent priority in the West only after we ourselves have come to feel threatened.

The recent airstrikes aimed at destroying the Islamic State’s sources of revenue are appropriate and should continue, provided they can be carried out without significant harm to innocent bystanders. But airstrikes against targets within the cities that the Islamic State controls will almost inevitably cause disproportionate harm to innocents. The West must not adopt the tactics of Assad. Ground forces capable of discriminating between Islamic State militants and civilians will almost certainly be necessary to dislodge the militants from the cities they control. And expelling them is essential, as the caliphate cannot exist in the eyes of its devotees in the absence of territorial control.

[Why what we call the Paris attackers matters]

Yet to send Western soldiers into combat against the militants would be perceived by Islamic State members and potential recruits as confirmation of prophesies in which they believe — fantasies involving the reappearance of Crusaders in their Holy Land. For this reason and others, it is important that the forces that purge the cities and towns should consist predominantly of soldiers indigenous to the region: Shiites, Kurds and, one hopes, Sunnis who detest the barbarism of their co-religionists in the Islamic State

This is one of many reasons why the resistance in the West, and most disgracefully in the U.S., which assisted in the creation of the Islamic State through its misbegotten war in Iraq, to providing asylum for refugees from Syria and Iraq is so profoundly wrong. It is probably true that a tiny fraction of the refugees are Islamic State infiltrators. But they will add only marginally to home-grown jihadists in the West who will become increasingly numerous until the Caliphate is destroyed by depriving it of territory. To turn away the principal victims of the Islamic State is not only inhumane but also self-defeating; for who are more likely to be our reliable allies in the morally necessary fight against the Islamic State than those whom it has driven from their homes and homeland?

Explore these other perspectives:

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.

Nolen Gertz: There are no more just wars

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