A Syrian policeman patrols the ancient oasis city of Palmyra in March. Many Syrian antiquities have been looted and smuggled out of the country during the past three years of civil war. Joseph Eid/AFP/Getty Images hide caption

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On the outskirts of Islamabad, a Pakistani health worker vaccinates an Afghan refugee against polio. Muhammed Muheisen/AP hide caption

itoggle caption Muhammed Muheisen/AP

An artist's rendering of the Crac des Chevaliers in Syria. UNESCO says the fortress "was built by the Hospitaller Order of Saint John of Jerusalem from 1142 to 1271." Hulton Archive/Getty Images hide caption

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A Syrian woman cries as she leaves a residential block in Aleppo, Syria, reportedly hit by an explosives-filled barrel dropped by a government forces helicopter on March 18. Khaled Khatib/AFP/Getty Images hide caption

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Rebel fighters inspect the wreckage of a Syrian army helicopter after the Islamic State of Iraq and Syria, or ISIS, allegedly destroyed it in March in the northern Syrian city of Aleppo. Mohammed Al-Khatieb/AFP/Getty Images hide caption

itoggle caption Mohammed Al-Khatieb/AFP/Getty Images

A man looks at an unexploded barrel bomb that landed in a cemetery after being dropped by forces loyal to Syria's President Bashar Assad in the northern Syrian city of Aleppo on Thursday. Reuters/Landov hide caption

itoggle caption Reuters/Landov

An informal Girl Scout group at the Zaatari refugee camp in Jordan sings: "We want to learn and rise up to fulfill our dreams." Nabih Bulos/NPR hide caption

itoggle caption Nabih Bulos/NPR