The early Islamic period in Iraq post the Arabian conquest
First major encounter between Arab and Sasanian forces took place in 634 AD and ended in Arabs being thoroughly routed. This conflict is called Battle of the Bridge or the Battle of al-Jisr. Abu Ubaid al-Thaqafi led Arab forces faced Bahman Jaduya led Persian Sasanian forces near present day Kufa. 5000 strong muslim forces were routed by Sasanian forces. Later on In 637 a comparatively larger Muslim force under Saʿd ibn Abī Waqqāṣ routed the main Persian army at the Battle of Al-Qādisiyyah and proceeded to sack Ctesiphon. By the next year, the Muslims had subjugated almost all of Iraq, and the last Sasanian king (Yazdegerd III) had fled to Iran, where he was murdered in 651.
The Muslim conquest led to mass immigration of Arabs from eastern Arabia and Oman. These new arrivals did not disperse and settle all over the country; on the contrary they established two new garrison cities, at Kūfah (near ancient Babylon) and at Basra in the south. The intention possibly was that the Muslims should be a distinct community of fighting men and their families living off taxes paid by the local inhabitants. In the north, Mosul began to develop as the most significant city and the base of a Muslim governor and garrison. Excepting the Zoroastrian priests and the Persian elite, whose properties were confiscated, most of the locals were permitted to keep their possessions and their religion.
Iraq was now a province of the Muslim Caliphate, which stretched from North Africa and later on Spain in the west to Sind (in present day southern Pakistan) in the east. Initially the capital of the Caliphate was at Medina, but, after the third caliph (ʿUthmān ibn ʿAffān) was murdered in 656, his successor (Prophet Muhammad’s cousin and son-in-law ʿAlī) made Iraq his base. Ironically in 661 ʿAlī was assassinated in Kūfah, and the Caliphate went into the hands of the Umayyad family (rival of the clan of Mohammad and Ali) in Syria. Iraq was now merely a subordinate province, despite being the wealthiest area of the Muslim world and the one with the largest Muslim population. This situation gave rise to persistent discontent with Umayyad rule that took various forms.
In 680 ʿAlī’s son al-Ḥusayn came to Iraq from Medina, anticipating support from the people of Kūfah. People of Kufah didn’t extend support to him, and his entourage was ambushed and massacred at the Battle of Karbala, but his remembrance lingered on as a source of motivation for all who opposed the Umayyads. In future the city of Karbala and ʿAlī’s tomb at proximate Najaf became important centres of Shiʿi pilgrimage still greatly revered.