For Iraq, the last years of the Sasanian empire were marked by civil war and a dramatic deterioration in the irrigation system. A vast area of southern Iraq revert to swampland (which it remained until the 20th century). It was into this situation that Arab armies swept in and swiftly conquered the country from the Sasanians (634-8).
As in other parts of the huge Islamic Caliphate, the conquered were allowed to remain Christians and keep their laws and customs. Huge numbers of Arabs did come into the country, but were settled in large garrison cities, at Al-Kufah and Al-Basra.
After the 660s, Iraq became a centre of opposition to the rule of the Umayyad caliphs, based in Syria. The corruption of Umayyad officials, and the neglect of the irrigation systems on which much of Iraq’s agriculture depended, led to economic depression, rising poverty and brigandage. Large numbers of Iraqi Muslims joined the Shi-ite sect, and these factors come together to give popular support in Iraq to a rebel army from Iran. In this year (750), the rebels defeat the Umayyad forces install a new caliph. He founds the ’Abbasid dynasty and chooses Iraq as his base.
ORIGINAL SOURCE: timemaps.com