Baghdad, founded in the 8th century, rapidly evolved into the capital of the Abbasid Caliphate and the central cultural hub of the Muslim world. Asōristān became the capital province of the Abbasid Caliphate and the center of the Islamic Golden Age for five hundred years. After the Muslim conquest, Asōristān saw a gradual but large influx of Muslim peoples; at first Arabs arriving in the south, but later also including Iranian (Kurdish) and Turkic peoples during the mid to late Middle Ages.

The Islamic Golden Age, a time of remarkable scientific, economic, and cultural progress in Islamic history, is traditionally dated from the 8th to the 13th century.^[49] This era is often considered to have commenced with the reign of Abbasid Caliph Harun al-Rashid (786-809) and the establishment of the House of Wisdom in Baghdad. This institution became a center of learning, attracting scholars from across the Muslim world to translate classical knowledge into Arabic and Persian. Baghdad, then the world's largest city, was a hub of intellectual and cultural activity during this period.^[50]

By the 9th century, however, the Abbasid Caliphate began to decline. During the late 9th to early 11th centuries, a phase termed the "Iranian Intermezzo," various minor Iranian emirates, including the Tahirids, Saffarids, Samanids, Buyids, and Sallarids, governed parts of what is now Iraq. In 1055, Tughril of the Seljuk Empire captured Baghdad, although the Abbasid caliphs continued to hold a ceremonial role. Despite losing political power, the Abbasid court in Baghdad remained highly influential, especially in religious matters. The Abbasids played a key role in maintaining the orthodoxy of the Sunni sect, in contrast to the Ismaili and Shia sects of Islam.

The Assyrian people continued to endure, rejecting Arabization, Turkification and Islamization, and continued to form the majority population of the north as late as the 14th century, until the massacres of Timur drastically reduced their numbers and led to the city of Assur being finally abandoned. After this period, the indigenous Assyrians became the ethnic, linguistic and religious minority in their homeland that they are to this day.