autonomous federation of the Roman Empire. Muslim conquest After the end of the Ridda wars in the Arabian Peninsula, Abu Bakr sent four armies to the Levant, led by Yazid ibn Abi Sufyan, Abu Ubaidah ibn al-Jarrah, Amr ibn al-Aas, and Shurahbil ibn Hasana. Because of the strength and size of the armies of the Byzantine Empire, Abu Bakr ordered Khalid ibn al-Walid to march with half of the Iraqi army to the Levant and command the armies there. Khalid set off with his army towards Sham and opened Bosra and then defeated the Byzantines at the Battle of Ajnadayn. After Umar ibn Al-Khattab became caliph in 13 AH (634 AD), Khalid was replaced by Abu Ubaidah ibn al-Jarrah. Abu Ubaidah was ordered to complete the conquest. He took Damascus, Baalbek, Homs, Hama and Latakia. After the successive defeats of the Byzantine army, the Byzantine Emperor Heraclius requested the help of the Arab Christians in Mesopotamia. They mobilized a large army and headed towards Homs, now the base of Abu Ubaidah in northern Syria, which they besieged. Heraclius also sent soldiers from Alexandria. Omar ibn al-Khattab wrote to Saad ibn Abi Waqqas to request support Abu Ubaidah with forces from Iraq, who were then organized under Iyad ibn Ghanm. When the Byzantines who were besieging Homs heard about the army coming from Iraq, they withdrew from Homs. Saad ordered Iyad to invade Upper Mesopotamia, which he conquered in 17 AH, including Deir Ez-Zor. At the time, Deir Ez-Zor were adherents of Christianity and Judaism. There was a Christian monastery in Monastery of the Hermits, which became Omari Mosque. Many of the town's Christians left or converted to Islam. During the Abbasid era, Deir Ez-Zor grew. The agriculture in the region prospered because of advances in irrigation. The small town, now called 'Deir Al-Rumman,' did not record any significant events during the decline of the Abbasid state and the ensuing Mamluk period until its destruction by the Mongols in the thirteenth century. Ottoman Era First Ottoman Era (1517–1864) The first Ottoman era extended from the date the Ottomans entered Syria in 1517 until 1864, where the Ottomans found Deir Ez-Zor a small town on the upper Euphrates and chose it as a center for their employees and settled in some of tribal sheikhs to protect the trade route between Aleppo and Baghdad and The tribe members began to visit it to