Baghdad and the golden age
Al-Mansur founded Baghdad in 762 as a round city on the Tigris. Within decades it was one of the largest cities on earth. Harun al-Rashid (786 to 809) and his son al-Ma'mun funded the House of Wisdom, where scholars translated Greek, Persian and Indian works into Arabic. Al-Khwarizmi wrote on algebra, and paper, a craft learned from Chinese prisoners after Talas, made books cheap.
Abu Hanifa, Malik, al-Shafi'i and Ahmad ibn Hanbal, the founders of the four Sunni schools of law, all lived in this period, as did the great hadith collectors al-Bukhari and Muslim.
Civil war and the Turkish guard
Harun's sons al-Amin and al-Ma'mun fought over the throne, and the war ended with Baghdad besieged and al-Amin killed in 813. Al-Ma'mun's mihna tried to force scholars to say the Quran was created, and Ahmad ibn Hanbal was flogged for refusing.
Al-Mu'tasim built an army of Turkish slave soldiers and moved the court to Samarra. Within a generation those soldiers were making and killing caliphs. Al-Mutawakkil was murdered in 861, and three more caliphs were deposed and killed in the decade that followed.
Rival caliphs and sultans
Provinces broke away: Spain in 756, then Egypt and North Africa. The Ismaili Fatimids declared a rival caliphate in 909 and founded Cairo in 969, and in Cordoba Abd al-Rahman III took the title in 929. In 945 the Buyids took Baghdad, and in 1055 the Seljuk Turks replaced them. The caliph was now a religious figurehead; the sultan ruled.
The Crusades
After the Seljuks crushed Byzantium at Manzikert in 1071, the emperor asked the Pope for help. The First Crusade took Jerusalem in 1099 with a massacre. The Muslim answer took decades: Zengi took Edessa in 1144, his son Nur al-Din united Syria, and Salahuddin, who ended the Fatimid caliphate in 1171, destroyed the crusader army at Hattin and retook Jerusalem in 1187.
The Third Crusade under Richard I took Acre but never Jerusalem. The Mamluks captured the last crusader fortress, Acre, in 1291.
The Mongols
Genghis Khan destroyed the Khwarazmian empire between 1219 and 1221. In 1258 his grandson Hulagu took Baghdad, killed the last caliph al-Musta'sim and massacred much of the city. The Mamluks of Egypt stopped the Mongols at Ain Jalut in 1260.
In 1261 Sultan Baybars seated a surviving Abbasid in Cairo as caliph. The office was symbolic, and it lasted until the Ottomans took Egypt in 1517.
Scholars of the age
These centuries produced many of Islam's greatest minds: the physicians al-Razi and Ibn Sina, the polymath al-Biruni, the philosopher al-Farabi and the theologian al-Ghazali, who taught at the Nizamiyya college in Baghdad founded by the Seljuk vizier Nizam al-Mulk. Much of their work later reached Europe in Latin translation and was studied in its universities for centuries.
Why the Abbasid caliphate ended
It did not end in a single day. The caliphate lost its provinces to local dynasties, its army to Turkish commanders and its authority to Buyid and Seljuk sultans long before 1258. The Mongols ended what remained in Baghdad, and the Ottomans ended its shadow in Cairo.
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