In the middle of the eighth century the Arabs had reached the zenith of their glory as a great conquering power. The succeeding period is one of much external magnificence, beneath which lurked the elements of corruption and decay. It opens with a division of the empire into two entirely separate and independent states. The contests of rival candidates for the caliphate had hitherto been decided after a short and sharp struggle, but so vast an increase of territory rendered it impossible for a single ruler to maintain his authority over a people divided into innumerable sects, each of which could put forward its own claimant to the seat of Mohammed. In 750 Merwan II., the last of the Omiades, was defeated on the banks of the Zab by Abul Abbas, who represented the descendants of Abbas, Mohammed's uncle. He attempted to exterminate: the rival family, but Abder Rahman, grandson of Hisham, escaped to Africa, and after obtaining succour from the Berbers, founded in 755 the 0miad dynasty of Spain. About 762 Bagdad, built by Mansur, son of Abul Abbas, became the capital of the eastern caliphate. Amid the splendours of this city his successors gave themselves up to a life of luxurious refinement, and the period of the early Abbassides is the most glorious in the annals of Arabic art, philosophy, and literature. The lust of conquest withstood for some time the enervating influence of an effeminate civilisation. Harun al Rashid (786-809), Mamun (813-33), and Motasim (833-42), carried war and devastation through the provinces of Asia Minor, and threatened Constantinople. But Mamun drained the life blood of Mohammedanism by supporting the Persian sceptics who disputed the inspiration of the Koran. The Arabs were further outraged by the appointment of Persians to the command of armies, and high offices of state. The same line of policy was pursued by his successors, and it proved fatal to the integrity of the empire. Motasim instituted a bodyguard of 70,000 Turks, who under Wathek (842-7) and his successors obtained the complete control of affairs, setting up and pulling down Caliphs at their will. During the remaining four centuries of its existence the eastern Caliphate was a scene of ever increasing anarchy and confusion. It would be impossible to enumerate all the sects and dynasties which at various times exercised a greater or less degree of sovereignty in regions nominally subject to the Caliph. The only method by which the ruler at Bagdad could hope to curb these dangerous adversaries was that of inviting the assistance of some powerful tribe on their borders, who made use of the opportunity to carve for themselves an empire out of his dominions. The Soffarides, who had made themselves independent in Korassan, were in 898 vanquished by Ismail Samana, king of Bokhara, who had invaded their territory at the request of the Caliph Motaded. The Samanades soon showed themselves as troublesome neighbours as the Soffarides had been. In order to conciliate the Turks, Radi (934-40) created the office of Emir-al-Omra, and into the hands of this minister he resigned all his temporal power. Even this step did not save him from ruin, for in 945 Bagdad was taken by the Buvides or Dilemites, who came from the neighbourhood of the Caspian. Both the Caliph and his vizier now lost all political influence, though the former was still regarded as the spiritual head of Islam. During the eleventh century the Gaznivedes spread themselves from Afghanistan over Persia and a portion of northern India. They were overthrown by the Seljuk Turks, who had in 1055 expelled the Buvides from Bagdad.
After the division of the empire in the eighth century the eastern Caliphs lost all influence in the Mohammedan Countries bordering on the Mediterranean. In 823 Crete was conquered by a band of Andalusian pirates, who kept possession of the island till it was retaken by the Emperor Nikephoros Phokas. During the ninth century the Aglabite dynasty, who had established themselves at Kairwan, overran a great part of Italy, attacked Rome (846), which was saved by the energy of Pope Leo IV., and in 878 completed their conquest of Sicily by the capture of Syracuse. This line of Caliphs was in 909 overthrown by Obeidalla, the representative of a dynasty which claimed to be descended from Ali and Mohammed's daughter, Fatima. The Fatimites or Shias fixed their residence at Mahadi, near Kairwan; in 970 they gained possession of Egypt, where they founded Cairo and continued to rule till they were overthrown by Saladin in 1171. Meanwhile the vigour of the Macedonian Emperors Nikephoros Phokas (963-73) and John Tzimiskes (973-6) had enabled them to recover the Byzantine dominions in Asia, which had become split up into a number of small Saracen states. But they never won back Syria, which was held by Fatimite Caliphs till the Turks conquered it during the latter part of the eleventh century.
The court of Cordova, the capital of the western Caliphate, rivalled in magnificence that of Bagdad, especially during the reign of Abd-er-Rahman the Third (912-61). The Omiades came to an end in 1031, but the title of Caliph was retained by their successors, the rulers of the Moorish dynasties of the Almoravides and the Almohades.
The Abbassides continued to reside at Bagdad till 1258, when the city was sacked by Hulaku, the grandson of Jenghis Khan. They then sought refuge in Egypt, where, under the protection of the Mamelukes, they retained their spiritual authority till 1577. Their title then passed to the Sultan of Constantinople.