We can't imagine the anguish of Kayla Mueller's family. We can try, but our thoughts skim off the surface. Perhaps, when news came that the 26-year-old aid worker had been killed in an airstrike in Syria, her parents initially comforted themselves with the thought that her final days had not been as bad as those of some of the Islamic State's hostages. It was reported, after all, that she had converted to Islam and been given a decent burial.

Now, even that consolation has been ripped away. An escaped Yezidi girl reports that the volunteer from Arizona, who had gone to Syria because she wanted to help its people, had been abused like the other captives, subjected to frequent rape and torture by Abu Bakr al-Baghdadi, the brigand chieftain who, blaspheming against his professed faith, presumes to call himself Caliph of Islam.

I'm wracking my brain for anything that the Islamic State could do to slide further along the spectrum of evil, and I'm struggling. Tony Abbott, the patriotic Australian leader, provoked faux-outrage the other day when he described the murderers as "worse than the Nazis," but he was absolutely right.

As he pointed out, the Nazis at least tried to hide the evidence of their crimes, because they couldn't quite shake their sense that what they were doing was ugly. But the Islamic State flagrantly behaves in as monstrous a way as it can, murdering charity workers, torturing civilians, dynamiting antiquities, enslaving children.

Two years ago, when the House of Commons voted to authorize bombing Syria, I opposed our involvement. I and my fellow non-interveners carried the day and, as a direct consequence, Washington, too, dropped the idea. But I was wrong. We were all wrong.

We were wrong because of a concept that is ill-suited to the snap judgments and 140-character summaries of the present era: that of the lesser evil. The arguments we made against airstrikes remain valid. There would be unintended consequences, collateral casualties. Everything would suddenly become our responsibility, and a chunk of international opinion would turn viciously against us.

All true. But look at the alternative: a humanitarian catastrophe that dwarfs anything seen in Iraq through the long years of unrest there; 11 million Syrians displaced from their homes, 4 million of them washing up as refugees abroad; friendly states in the region destabilized; and Western-born Muslims drawn into the vortex. How much worse do things have to get, for Heaven's sake?

A choice between bad and worse defies the self-righteous simplicity that our age demands. Politicians shy away, and you can see why. Suppose, for the sake of argument, that the Clinton administration had decided to intervene in Rwanda in 1994. All sorts of grim consequences might have followed.

American troops could have been drawn into a Somalia-type quagmire, drawing anti-war crowds onto the streets. President Clinton might well have lost the 1996 election. Had he tried to argue that his intervention, with all its faults, had prevented the largest genocide of the post-war era, he'd have been written off as a madman.

Many of us anti-interveners now feel that we're stuck on a hook; but, really, the situation has changed utterly since August 2013. Then, the Islamic State barely existed. Now, Syria's options are so gruesome that many moderate states in the region find themselves backing militias affiliated with al-Qaeda — not out of any ideological affinity but because, between Assad and the Islamic State, they have become the least bad option.

The Islamic State has kidnapped and murdered U.S. and British citizens for no other reason than that they hold our passports. That, by almost any definition, is an act of war, inviting retaliation. Yet still we wriggle on with the anti-intervention barbs on which we impaled ourselves in 2013.

I am not suggesting an Iraq-style invasion. But there is now a strong case for targeted airstrikes, and for the establishment of safe areas, protected by international forces, rather as when the Kurdish part of Iraq broke away from Saddam. The mass of Syrians who support neither Assad nor the Islamist gunmen have never been given a chance. They were defeated, not at the ballot box, but on the battlefield.

Several neighboring Arab states are ready to join a coalition, and Turkey has indicated its willingness to commit ground forces to create a northern safe zone. They are waiting only for our lead. Why us? Because, in the end, it's always us, the English-speaking peoples. And we're not done yet.

This article appears in the Sept. 14 edition of the Washington Examiner magazine.

Dan Hannan is a British Conservative MEP.