Omar I. (Abu Hafsah Ibn Al-Khattab), the second of the caliphs, third cousin of Ab-dallah, the father of Mohammed, born about 581, assassinated in 644. Originally an enemy of the prophet, he set out for Medina to murder him; but chancing to read what is now the 20th chapter of the Koran, he was converted, and thenceforth became a zealous supporter of the Moslem faith. "When the prophet died in 632, Omar resigned his claim to the supreme rule in behalf of Abubekr; and the latter on his deathbed appointed him his successor (634). During his reign the Moslems were everywhere victorious. One army prosecuted the conquest of Syria, took Damascus in 635, defeated the Greeks in the bloody battle of Yermuk, and in 637 compelled Jerusalem to capitulate. Another army under Amru invaded Egypt, reduced Alexandria, and overran Africa as far as the deserts of Tripoli and Barca. In Persia and Armenia the Moslems were also successful. A Mohammedan historian sums up the conquests of Omar by saying: "He took from the infidels 36,000 cities or castles, destroyed 4,000 temples and churches, and founded or endowed 1,400 mosques." In the beginning of November, 644, while performing his morning devotions in the mosque at Medina, he was stabbed three times by a Persian slave named Firuz, who had applied to him in vain to be relieved of half the tribute he was compelled to pay his master.

He lingered five days, and refused to name his son as his successor. Omar's only food was barley bread and dates; his only drink, water; and his garment an old coat torn in twelve places. He regularly distributed the contents of his treasury among his soldiers and followers, and supported himself by the work of his own hands, manufacturing and selling leather belts. He was the first to assume the title of emir el-mumenim, or commander of the faithful. In his time the era of the Hegira was established, armies were first kept under pay, and a sort of police force was instituted. Omar has always been an object of reverence among the Moslems of the Sunni or orthodox sect. He was succeeded by Othman.