The 'Abbasid dynasty (750–1258)

Non-Arab converts to Islam, who were taxed and treated as second-class citizens under Umayyad rule despite Islamic teaching on equality.

A secret organisation built in Khurasan from around 718 that recruited support for 'a member of the family of the Prophet' without revealing it would be an 'Abbasid, not a Shi'a Alid.

Abu Muslim, a general of largely mawali background who raised the black-bannered army in Khurasan in 747.

Abu'l-'Abbas al-Saffah, proclaimed in the mosque at Kufa in 749.

The decisive battle where the 'Abbasid army destroyed the main Umayyad force; Caliph Marwan II fled and was later killed in Egypt.

Arab tribal favouritism over mawali; Shi'a resentment since Ali's death (661); Qaysi-Yamani tribal feuding; weak Umayyad control over distant Khurasan.

Almost all were massacred, including at a famous banquet; one survivor, Abd al-Rahman, escaped to Spain and founded a separate Umayyad emirate at Córdoba.

Damascus was the Umayyads' Arab tribal power base; Iraq was central, wealthy, and home to the mawali/Persian-influenced groups who had backed the 'Abbasid cause, so moving there consolidated the new regime's support.

762, by the second 'Abbasid caliph, al-Mansur.

He was executed in 755 on the order of Caliph al-Mansur, who feared his popularity and independent power base in Khurasan.

They had expected a descendant of Ali (an Alid) to become caliph, but the 'Abbasids — a different branch of the Prophet's extended family — took power instead.

A reasoned judgement weighing the relative importance of different factors against each other, not just a list or narrative of causes.

Al-Mansur, the second 'Abbasid caliph, founded Baghdad in 762 as the new imperial capital, shifting power east into Iraq.

Abu Muslim had commanded the Khurasani army that won the 'Abbasid Revolution, making him powerful enough to threaten al-Mansur's throne.

The height of 'Abbasid wealth, trade and culture, with Baghdad as a major world city — though he purged the Barmakid viziers and split succession between his sons, sowing future conflict.

Al-Ma'mun won a civil war against his half-brother al-Amin in 813, after Harun al-Rashid had divided succession between them.

A major Baghdad institution, founded under al-Ma'mun, for translating and studying Greek, Persian and Indian scholarly texts.

An inquisition-like religious test imposed by al-Ma'mun to enforce Mu'tazilite theology on scholars, causing conflict with the Ulama.

Al-Khwarizmi (mathematics — founded algebra) and al-Razi (medicine — clinical observation and hospital care).

Political stability, trade wealth, and deliberate state patronage of scholars, building on translated Greek/Persian/Indian knowledge.

Turkic slave-soldiers the caliphs relied on for their armies grew so powerful that their commanders began appointing and deposing caliphs themselves.

The Fatimid Caliphate in Cairo (from 909) and the Umayyad Caliphate of Spain, both claiming to be legitimate leaders of the Muslim world.

Hulagu Khan's Mongol army besieged and sacked Baghdad, destroyed the House of Wisdom, and executed the last 'Abbasid caliph in Iraq, al-Musta'sim.

Under Harun al-Rashid the caliph directly ruled a wealthy, unified empire; by 1258 real power lay with Seljuk sultans and provinces had broken away, leaving the caliph a figurehead before the Mongols ended the dynasty entirely.