The Umayyads fell from power in 750 because of a revolutionary uprising against their rule led by the Abbasids, a clan descended from Muhammad's uncle. The Abbasids were supported by many non-Arab but Muslim subjects of the Caliphate (called mawali) who resented the fact that the Umayyads had always protected the status of Arabs at the expense of non-Arab Muslims in their empire. After seizing control of the Caliphate, the Abbasids went on a concerted murdering spree, trying to eliminate all potential Umayyad competitors, with only a single member of the Umayyad leadership surviving. The Abbasids lost control of some of the territories that had been held by the Umayyads (starting with Spain, which formed its own caliphate under the surviving Umayyad), but the majority of the lands conquered in the Arab conquests a century earlier remained in their control.
The true golden age of medieval Islam took place during the Abbasid Caliphate. The Abbasids moved the capital of the caliphate from Damascus to Baghdad, which they founded in part to be nearer to the heart of Persian governmental traditions. There, they combined Islam even more closely with Persian traditions of art and learning. They also created a tradition of fair rulership, in contrast to the memory of Umayyad corruption. The Abbasid caliphs were the leaders of both the political and spiritual orders of their society, seeking to make sure everything from law to trade to religious practice was running smoothly and fairly. They oversaw fair trade practices and used their well-trained armies primarily to ensure good trade routes, to enforce fair tax collection, and to put down the occasional rebellion. The Abbasid rulers represented, in short, a kind of enlightened despotism that was greatly ahead of Byzantium or the Latin kingdoms of Europe in terms of its cosmopolitanism. The Abbasids abandoned Arab-centric policies and instead adopted Muslim universalism that allowed any Muslim the possibility of achieving the highest state offices and political and social importance.
Perhaps the most important phenomenon within the Abbasid caliphate was the great emphasis and respect the caliphs placed on learning. New discoveries were made in astronomy, metallurgy, and medicine, and learned works from a variety of language were translated and preserved in Arabic. The most significant tradition of scholarship surrounding Aristotle's works, in particular, took place in the Abbasid caliphate.
The major library in Baghdad was called the House of Wisdom; it was one of the great libraries of the world at the time. The various advances that took place in the Abbasid Caliphate included:
- Medicine: far more accurate diagnoses and treatments than existed anywhere else (outside of China).
- Optics: early telescopes, along with the definitive refutation of the idea that the eye sends out beams to detect things and instead receives information reflected off of objects.
- Chemistry: various methods including evaporation, filtration, sublimation, and even distillation. Despite the specific ban on intoxicants in the Koran, it was Abbasid chemists who invented distilled spirits: al-kuhl, meaning “the essence," from which the English word alcohol derives.
- Mathematics: the creation of Arabic numerals, based on Hindu characters, which were far easier to work with than the clunky Roman equivalents. In turn, the Abbasids invented algebra and trigonometry.
- Geography and exploration: accurate maps of Asia and East Africa, thanks to the presence of Muslim merchant colonies as far as China, along with new navigational technologies like the astrolabe (a device that is used to determine latitude while at sea).
- Banking: the invention of checks and forms of commercial insurance for merchants.
- Massive irrigation systems, which made Mesopotamia nearly on par with Egypt as the richest farmland in the world.
In addition, the Abbasid Caliphate oversaw a major increase in literacy. Not only were Muslims (men and women alike) encouraged to memorize the Koran itself, but scholars and merchants were often interchangeable; unlike medieval Christianity, Islam did not reject commerce as being somehow morally tainted. Thus, Muslims, whose literacy was due to study of specifically Islamic texts, the Koran and the Hadith especially, easily used the same skills in commerce. The overall result was a higher literacy rate than anywhere else in the world at the time, with the concomitant advantages in technological progress and commercial prosperity.
At its height, the Abbasid Empire was truly enormous– it covered more land area than had the Roman Empire. Its merchants traveled from Spain to China, and it maintained diplomatic relations with the rulers of territories thousands of miles from Baghdad. The Caliphate reached its peak during the rule of the caliph Harun al-Rashid (r. 786 – 809). His palace was so enormous that it occupied one-third of Baghdad. He and the greatest early-medieval European king, Charlemagne, exchanged presents and friendly letters, albeit out of political expediency: Charlemagne was the enemy of the Cordoban Caliphate of Spain, the last vestige of Umayyad power, and the Abbasids acted as an external pressure that Charlemagne hoped would make the Byzantine emperors recognize the legitimacy of his imperial title (as an aside, one of Charlemagne’s prized possessions was his pet elephant, sent to his distant court by al-Rashid as a goodwill gift).
Already by al-Rashid’s reign, however, the Caliphate was splintering; it was simply too large to run efficiently without advanced bureaucratic institutions. North Africa west of Egypt seceded by 800, emerging as a group of rival Islamic kingdoms. Other territories followed suit during the rest of the ninth century, leaving the Caliphate in direct control of only the core lands of Mesopotamia. Within its remaining territory the caliphs faced uprisings as well. Even the idea of a united (Sunni) Ummah was a casualty of this political breakdown - the ruler of the Spanish kingdom claimed to be the “true” caliph, with a Shia dynasty in Egypt known as the Fatimids contesting both claims since it rejected the very idea of a Sunni caliph.
The political independence of the Caliphate ended in 945 when it was conquered by Persian tribesmen, who took control of secular power while keeping the Caliph alive as a figurehead. In 1055, a Turkish group, the Seljuks (the same group then menacing Byzantium), seized control and did exactly the same thing. For the next two centuries the Abbasid caliphs enjoyed the respect and spiritual deference of most Sunni Muslims, but exercised no political power of their own.
As Seljuk power increased, that of the Caliphate itself waned. Numerous independent, and rival, Islamic kingdoms emerged across the Middle East, North Africa, and northern India, leaving even the Middle Eastern heartland vulnerable to foreign invasion, first by European crusaders starting in 1095, and most disastrously during the Mongol invasion of 1258 (under a grandson of Genghis Khan). It was the Mongols who ended the Caliphate once and for all, murdering the last caliph and obliterating much of the infrastructure built during Abbasid rule in the process.