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.
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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
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
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
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.