Edessa ĭdĕs´ə [key], ancient city of Mesopotamia, on the site of modern Şanlıurfa, Turkey. It emerged in the 4th cent. BC as Orrhoe, or Arrhoe, and was later named Edessa by Seleucus I of Syria. From c.137 BC it was the capital of the independent kingdom of Osroene. It later became a Roman city. There in AD 260, Shapur I of Persia defeated Emperor Valerian and took him prisoner. Edessa was a center of Christianity by the 3d cent. AD and became one of the major religious centers of the Byzantine Empire. The city fell to the Arabs in 639 and remained in Muslim hands until captured by the Crusaders in 1098. Baldwin (later Baldwin I of Jerusalem) became the ruler of Edessa, and when he became king, he turned it over to one of his cousins. The city, however, fell to the Muslims in 1144 and passed to the Ottoman Empire by 1637.
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