SONS OF AL-RASHID
Civil War between Iraq and Iran
If Harun had tried to avoid conflict between his sons, his project failed. Al-Mamun had already been made governor of Khorasan - and was immediately accepted by the independent-minded Khorasanians as one of their own - especially because his mother was an Iranian: they called him "son of our sister". The rebels whom al-Mamun had come to fight surrendered and were pardoned. Al-Mamun's court began talking about a revival of Iranian culture - and even of Zoroastrianism. The mood was anti-Arab and anti-Islam.
In 810 al-Amin sacked his half-brother, and demanded full control of his provinces. He announced that his own son, Musa, was now the heir to the caliphate, and sent an army against Khorasan. In the end he sent three armies: the first two were defeated by al-Mamun's excellent general Tahir ("the ambidextrous"). The third returned to find a revolt in Baghdad itself (812) - which after a long siege fell to Tahir (813). Al-Amin was beheaded, and al-Mamun became caliph. He did not move to Baghdad, however, and put Tahir in charge of the western provinces. This was not a success, and resistance began to grow; there were numerous Shi'a inspired revolts. In 819 he finally entered Baghdad, and moved Tahir (who was becoming worryingly powerful) back to Khorasan, where he died. Al-Mamun had to recognise Tahir's son as ruler of Khorasan - the first breakaway dynasty, the Tahirids, was being established.
Al-Mamun in Baghdad: the Golden Age
Under al-Mamun Baghdad was at its most brilliant. Poetry (the chief of the arts to the Arabs) flourished as never before (inspired by the innovations of Abu Nawas) - but so did philosophy, theology, astronomy, medicine and law. He - or his father - are often credited with founding of a "House of Wisdom" (Bait al-Hikma). This was almost certainly not a "university" dedicated to translating dedicated to translation. Nevertheless, much translation of foreign, especially scientific and philosophical Greek works - Aristotle, Plato, Hippocrates and Galen - was taking place at this tim, encouraged by the Abbasid caliphs. In mathematics there were world-changing discoveries: the concept of zero, algebra, and trigonometry .
An embassy of scholars to Mamun (on right) from Byzantine emperor Theophilus (on left)
Mamun was also interested in religious reform - his intellectual qualities and his admiration for Greek philosophers led him to promote a variant version of Islam, known as Mu'tazilism. The Mutazilites wanted an Islam founded on reason and logic: in particular they argued that if the Quran is God's word, it could not have existed until God created it. Similarly evil was not something defined by scripture, but right and wrong, and thus justice, could be established through reason. Mamun somewhat undermined his position by instituting a kind of inquisition (Mihna) against those who disagreed with him. His campaigns against Christian Byzantium in the last years of his reign were actually a more effective way of uniting Islam!