ŞANLIURFA, Turkey—With its history going back 11,000 years, Şanlıurfa, Turkey, is one of the world’s oldest continuous settlements. The city is the capital of Şanlıurfa Province, an area with nearly 2 million residents. Sharing its entire 139-mile southern border with Syria, the region has become a violent frontier in the deadliest civil war in recent history. Along Turkey’s south border, four cities—Şanlıurfa, Gaziantep, Kilis, and Hatay—have deep interactions with the opponents to the regime of President Bashar al-Assad, including radical groups like the Islamic State of Iraq and Syria, or ISIS. The remaining border that southeastern Turkey shares with Syria lies near the cities of Mardin, Şırnak, Diyarbakır, and Hakkari. But this stretch is controlled by the Kurdistan Worker’s Party (PKK) and the Kurdish Democratic Union Party (PYD), and so little flows from Turkey to extreme jihadist groups here.

The population...

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