The Abbasid Caliphate ruled the Islamic world from 750, moved its capital to Baghdad, presided over the greatest flowering of medieval science and letters, and lost real power long before the Mongols ended it in 1258.
The Abbasids took the caliphate from the Umayyads in 750 and held the title for five centuries, but the story divides sharply in two. For roughly its first hundred and fifty years the dynasty governed the largest and richest state in the world; for the three centuries after that it reigned over an empire that had come apart underneath it.
The change of dynasty was also a change of civilisation. The capital moved from Damascus, a Mediterranean city in former Roman territory, to Iraq and a new foundation at Baghdad, and the administration took on the shape of Persian imperial practice — a vizier, a bureaucracy, a court.
For this archive the Abbasids are the eastern power that the Byzantine empire faced for its whole middle period: the enemy of the annual raids, the state that exchanged prisoners and embassies with Constantinople, and the one whose collapse in the 860s made the Byzantine recovery possible.
The Umayyads had ruled from Damascus as an Arab military aristocracy, and by the 740s their support had narrowed to the point of brittleness. The Abbasid movement — descended from the Prophet's uncle al-Abbas — gathered the disaffected: non-Arab Muslims excluded from privilege, Shi'a groups hoping for a family closer to the Prophet's line, and the discontented armies of Khurasan.
The revolt broke out in Khurasan in 747 under the black banners and swept west. The Umayyad army was destroyed at the river Zab in 750 and the dynasty was hunted down; one prince escaped to Spain and founded the emirate that became the Caliphate of Córdoba.
The new regime disappointed most of the coalition that had raised it. The Shi'a expectations were not met, and the Abbasids governed as an imperial dynasty rather than a restoration — but the barrier between Arab conquerors and everyone else was gone for good.
Al-Mansur founded Baghdad in 762 on the Tigris as a round city with the palace and mosque at its centre, and within fifty years it was probably the largest city in the world outside China. It sat on the trade routes between the Mediterranean, the Indian Ocean and Central Asia, and taxed all of them.
The reign of Harun al-Rashid (786–809) is the conventional high point, and his court entered European memory through the Thousand and One Nights — which is a later literary construction and not evidence for how he governed. What is solid is the machinery: a vizierate held for a generation by the Barmakid family, a professional bureaucracy, and a treasury with the reach to fund it.
The most durable achievement was intellectual. Under al-Ma'mun in particular, Greek, Persian and Indian works were systematically translated into Arabic, and the scholarship built on them in medicine, astronomy, mathematics and philosophy travelled back into Latin Europe centuries later. The Arabic transmission is how much of Greek science survived at all.
Al-Saffah (750–754) took the throne in the revolution. His brother al-Mansur (754–775) was the real founder of the state: he built Baghdad, crushed the rivals who had helped him to power, and set the dynasty's administrative shape.
Harun al-Rashid (786–809) presided over the height of Abbasid wealth and prestige and divided the succession between his sons, which produced a civil war. Al-Ma'mun (813–833) won it, patronised the translation movement, and imposed the mihna, an inquisition into religious doctrine that his successors abandoned.
Al-Mu'tasim (833–842) built the Turkish slave-soldier corps that became the state's military spine and eventually its master, and founded Samarra to house them. Al-Mutawakkil (847–861) built the Great Mosque there and was murdered by those same troops, opening the anarchy at Samarra. Al-Musta'sim (1242–1258) was the last caliph in Baghdad, killed by the Mongols.
The decline was not conquest from outside but the failure of the centre. Al-Mu'tasim's Turkish regiments were loyal to their commanders rather than to the dynasty, and after the murder of al-Mutawakkil in 861 they made and unmade four caliphs in nine years — the crisis known as the anarchy at Samarra.
Provinces stopped waiting for permission. The Aghlabids in North Africa, the Tulunids in Egypt, the Samanids in Central Asia and eventually the Hamdanids in northern Syria governed and taxed on their own account while acknowledging the caliph in the coinage and the Friday prayer.
The formal end of Abbasid power came in 945, when the Buyids took Baghdad and made the caliph a religious figurehead under military protection. The Seljuk Turks replaced them in 1055 in the same role. The title kept its authority; the office had none.
For Constantinople the Abbasids were the permanent eastern adversary, but a different one from the Umayyads. The capital was now in Iraq, not Syria, and no Abbasid caliph attempted to take Constantinople.
What continued was the frontier: the Thughur, a belt of fortified cities from Cilicia to the Euphrates whose emirs raided into Anatolia most years. Harun al-Rashid campaigned there in person and extracted tribute from the empress Irene; the emirate of Melitene under Umar al-Aqta made the raiding an annual terror in the ninth century.
The relationship had a civil register too. The two states exchanged embassies and negotiated prisoner exchanges at the river Lamos in Cilicia at intervals through the ninth and tenth centuries, and Byzantine and Abbasid scholarship crossed the frontier more easily than armies did.