[in full Omar ibn al-Khaṭṭab]. The second of the Mahommedan caliphs. Originally opposed to Mahomet, he became later one of the ablest advisers both of him and of the first caliph, Abū-Bakr. His own reign (634644) saw Islams transformation from a religious sect to an imperial power. The chief events were the defeat of the Persians at Kadisiya (637) and the conquest of Syria and Palestine. The conquest of Egypt followed (see Amr ibn al-Āṣ) and the final rout of the Persians at Nehāwend (641) brought Iran under Arab rule. Omar was assassinated by a Persian slave in 644, and though he lingered several days after the attack, he appointed no successor, but only a body of six Muhajirun who should select a new caliph. Omar was a wise and far-sighted ruler and rendered great service to Islam. He is said to have built the so-called Mosque of Omar (the Dome of the Rock) in Jerusalem, which contains the rock regarded by Mahommedans as the scene of Mahomets ascent to heaven, and by the Jews as that of the proposed sacrifice of Isaac.