One of the most heinous of the endless war crimes of the Islamic State has been the systematic rape of thousands of young girls and women - who are sold as sex slaves.

Most of the victims come from the Yazidi religious minority, labeled nonbelievers by ISIS. They were captured when ISIS invaded northern Iraq last year and wiped out their communities.

But one of the sex slaves was a fresh-faced blond American, a 25-year-old aid worker who was captured in Syria in August 2013. Kayla Mueller was chained in a room and raped for months by the leader of ISIS, Abu Bakr al-Baghdadi, before being killed in February (supposedly by a Jordanian air strike, but the true cause is uncertain).

What astonishes me is the paucity of global outrage at the buying and selling of sex slaves - not to mention U.S. outrage at the enslavement of Mueller. American women organized to protest the Taliban's repression of women but not ISIS atrocities that make the Taliban's war crimes look mild in comparison. How can this be?

It's not because ISIS's slave trade isn't heinous or heartbreaking. The New York Times' Rukmini Callimachi recently documented ISIS's revival of slavery as an institution, with a bureaucracy of warehouses and viewing rooms where Yazidi girls are auctioned as chattel.

ISIS has compiled Quranic scripture that sanctions slavery and sexual assault on nonbelievers (Callimachi interviews a 12-year-old escapee who recalls how her rapist prayed before and after he defiled her). Clearly there is a need for Muslim leaders and senior clerics worldwide who haven't already done so to label such scripture as centuries outdated, and denounce the practice of slavery.

As for the broader public, why no outcry? I asked Samer Muscati, senior researcher on women's rights for Human Rights Watch. He had recently interviewed 20 young Yazidi women in Iraq who had escaped ISIS slavery. He says these were among the worst cases he has ever documented - even after years of reporting on human-rights crimes in Iraq.

"People become numb to all these atrocities," he said sadly. "There is war fatigue, with nothing but doom and gloom from Iraq and Syria." He added that because people don't know much about the Yazidi minority, "there hasn't been the outcry of disgust you would expect." Moreover, Muscati says, it's hard for human-rights groups to "name and shame" a group that glorifies its own abuses, and acts outside any norms.

But what about Mueller's case? Here was a young American woman - kept as the personal sex slave of ISIS's leader - who wrote in a smuggled letter that she would never "give in" to her captors. She refused a chance to escape lest she endanger Yazidi girls imprisoned with her to whom she had become a mother figure. A U.S. effort to free her failed, as she had already been removed from the site that American commandos raided.

Yet her tragic story has caused hardly a ripple in U.S. public opinion.

Perhaps Americans are perplexed that an American would risk her life to try to help Syrians. Or maybe most Americans are simply confused about what, if anything, they can do to halt ISIS atrocities in the Mideast.

I can understand that confusion. President Obama's policies for fighting ISIS are so inept and contradictory they have helped the group's so-called caliphate to sink ever-deeper territorial roots. The White House still doesn't seem to recognize the long-term security threat the group poses to the U.S. homeland, as ISIS inspires ever more disgruntled youths to adopt its fanatic values.

Top U.S. military brass label a revanchist Russia the most urgent U.S. security challenge in the near term. But, if left unchecked, ISIS may pose the greater threat to the West over the next decade. And should anyone doubt the more immediate ISIS challenge to Europe, one needs only watch scenes of Syrian refugees streaming through the Balkans to escape the chaos that jihadis (and Damascus) are wreaking.

So the fight against sexual slavery must be part of a bigger struggle. This will require (finally) a serious U.S. policy to help Iraqis and Syrians who want to roll back the jihadis. Washington must give all needed military support to Iraqi, Syrian, and Turkish Kurds, who have fought most effectively against ISIS. It was the Kurds who pushed these fiends back from the Yazidi heartland, and any current U.S. effort to woo Turkey must not betray Kurdish fighters.

In the meantime, concerned Americans should press the administration to provide more targeted aid for Yazidi girls who have escaped ISIS, and are now living as refugees in Iraqi Kurdistan, whose regional government is overwhelmed and can't provide all the assistance that is needed.

"The girls and women who've escaped need access to rehabilitation and skills training," says Muscati, "and there is a real lack of psychological help" to enable them to cope, in a society where sexual abuse and abortion are taboo subjects. This is where concerned Americans can focus immediate attention.

The enslavement of Yazidi women symbolizes a movement that rejects every norm of civilization, and must be combated. For those who are outraged at ISIS's adulation of rape, there are things to be done.