IN THE SUMMER of 2014 — August, the sun hammering all day on prickly wheat fields — the first Islamic State fighters arrived in Sinjar province, close to the Syrian border in northwest Iraq.

Bearded and mostly young, lugging an array of weapons, they came in pickup trucks and cars, as well as Humvees captured from the Iraqi Army in earlier battles. Soon the black-and-white flag of ISIS, the freshest desert nightmare, would fly along roads and atop buildings in places that rarely (or never) make it onto American television: Tel Azer, Kojo, Siba Sheikheder, Tel Banat, Sinjar City. This last place was home of the Yezidis, a religious minority long accustomed to violent persecution.

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