For those who don't make it all the way through the article, here's the powerful ending:

As they surveyed the area afterward, Mr. Barash encountered several bodies of ISIS fighters that the militants had not claimed. Standing over one, Mr. Barash heard a phone ring from inside the insurgent’s pocket. He grabbed the phone and spoke: “Come and take your body.”

But an old man answered, weeping. He told Mr. Barash that ISIS had taken his son from him when they swept through his village. The militants had given him a choice: He could give them his daughter or one of his sons.

Crying on the phone, the old man said his son was a teenager, not even old enough for facial hair, and never learned how to fight.

“I told him I was sorry,” Mr. Barash said.