The heads of the academies in Babylon became the incontestable leaders of the Jewish world.
The major achievement of the medieval Babylonian geonim (8th to early 11th century) was the successful promotion of the Babylonian Talmud as the definitive rabbinic source regarding Jewish religious practice. Ironically, the widespread acceptance of the Babylonian Talmud (combined with the decline of the Baghdad caliphate and the impoverishment of Babylonian Jewry) contributed to the demise of the Babylonian geonim, as important, independent centers of talmudic learning emerged in the Diaspora. Reprinted with permission from A Historical Atlas of the Jewish People, published by Schocken Books.
The Muslim conquests were an important agent of unification for the Jewish communities throughout the diaspora. From the seventh century onwards, the vast majority of Jews were under single rule and part of a large network of commercial ties connecting the different sectors of the Muslim empire. After the Arabs conquered the Maghreb ("the west" in Arab geographic terminology, designating North Africa and Spain), thousands of Jews immigrated there, mostly from the east (particularly from the areas of Iraq and Iran of today).
The Jewish demographic map reflected a diversity largely due to incessant migrations. Nevertheless, there was a stable framework--a central authority which delegated some of its prerogatives to each community. Existing prior to the emergence of Islam, this structure was consolidated when the Muslim caliphate embraced a world of immense dimensions, obeying first Damascus and then Baghdad.
The seat of spiritual authority of the Jewish world was the yeshivah (academy). Between the eighth and eleventh centuries, this was not simply a learning institute, but also the supreme court and source of instruction for all Jews. The head of the yeshivah, the gaon, was regarded as the highest religious authority, but his responsibilities also included organizing the courts, appointing judges and community leaders as well scribes, ritual slaughterers and other officials. The gaon was authorized to dismiss any one of these, and it was he who exercised the powerful weapon of excommunication.
The geonim became incontestable leaders of the Jewish world as a result of two developments. The first was the conflict between the heads of the academies and rosh ha‑golah (the exilarch) who officially represented the Babylonian community to the authorities. The two Babylonian academies, the yeshivah of Sura and the one in Pumbedita, were transferred to Baghdad in the early ninth century and continued to carry the names their former locations. Before then the exilarch had held full authority and he nominated the heads of the academies who were regarded as spiritual leaders only. But in time, the heads of the academies acquired more power; the office of exilarch persisted, but essential secular functions were taken over by the geonim.