The Jews in the ancient Babylonian district (called Irak by the Arabs) attained a great measure of freedom through the victories of the Mahometans. During their campaigns against the last Persian kings, the Jews and the Nestorian Christians, who had been persecuted under the last Sassanian princes, had rendered them much assistance. The Jews and the Chaldean Christians formed the bulk of the population near the Euphrates and the Tigris. Their assistance must have been opportune, as we find even the fanatical Caliph Omar bestowing rewards and privileges upon them. It was, doubtless, in consequence of the services which they had rendered that the Mahometan generals recognized Bostanaï, the descendant of the Exilarch of the house of David, as the chief of the Jews. Omar respected Bostanaï so highly that he gave him a daughter of the Persian king Chosru in marriage. She had been taken prisoner, together with her sisters (642)--a singular turn of fate! The grandson of a race that boasted descent from the house of David married a princess whose ancestors traced their descent from Darius, the founder of the Persian dynasty. Bostanaï was the first Exilarch who was the vassal of the Mahometans. The Exilarch exercised both civil and judicial functions, and all the Jews of Babylonia formed a separate community under him. Bostanaï also obtained the exceptional permission to wear a signet-ring (Gushpanka). By this means he was able to give his documents and decrees an official character. The seal, in reference to some unknown historical allusion, bore the impress of a fly. Bostanaï must have been an important personage in other respects, since legends cluster about him, and would make his birth itself appear a miraculous event. The Judæo-Babylonian community, which had acquired some importance through Bostanaï, obtained its real strength under Ali, the fourth Caliph, Mahomet's comrade and son-in-law, the hero of Chaibar. Omar had died at the hands of an assassin (644), and his successor, Othman, had been killed in an insurrection (655). Ali was nominated Caliph by the conspirators, but he had to struggle against many bitter opponents. Islam was divided into two camps. The one declared for Ali, who resided in the newly-built town of Kufa; the other for Moawiyah, a relative of the murdered Caliph Othman.