Iraq from c. 600 to 1055
In 600 Iraq was a province of the Persian Sāsānian empire, to which it had belonged for three centuries. It was probably the most populous and wealthy area in the Middle East, and the intensive irrigation agriculture of the lower Tigris and Euphrates rivers and of tributary streams such as the Diyālā and Kārūn formed the main resource base of the Sāsānian monarchy. The name Iraq was not used at this time; in the mid-6th century the Sāsānian empire had been divided by Khosrow I into four quarters, of which the western one, called Khvarvaran, included most of modern Iraq.
The name Iraq is widely used in the medieval Arabic sources for the area in the centre and south of the modern republic as a geographic rather than a political term, implying no precise boundaries. The area of modern Iraq north of Tikrīt was known in Muslim times as Al-Jazīrah, which means “the Island” and refers to the “island” between the Tigris and Euphrates rivers (i.e., Mesopotamia). To the south and west lay the Arabian Desert, inhabited largely by Arab tribesmen who occasionally acknowledged the overlordship of the Sāsānian kings. Until 602 the desert frontier had been guarded by the Lakhmid kings of Al-Ḥīra, who were themselves Arabs but ruled a settled buffer state. In that year Khosrow II (Parvīz) rashly abolished the Lakhmid kingdom and laid the frontier open to nomad incursions. Farther north the western quarter was bounded by the Byzantine Empire. The frontier more or less followed the modern Syria-Iraq border and continued northward into modern Turkey, leaving Nisibis (modern Nusaybin) as the Sāsānian frontier fortress while the Byzantines held Dārā and nearby Amida (modern Diyarbakır).
The inhabitants were of mixed background. There was an aristocratic and administrative Persian upper class, but most of the population were Aramaic-speaking peasants. A considerable number of Arabs lived in the region, most of them as pastoralists along the western margins of the settled lands but some as townspeople, especially in Al-Ḥīra. In addition, there were Kurds, who lived along the foothills of the Zagros Mountains, and a large number of Greeks, mostly prisoners captured during the numerous Sāsānian campaigns into Byzantine Syria.
Ethnic diversity was matched by religious pluralism. The Sāsānian state religion, Zoroastrianism, was largely confined to the Persian ruling class. The majority of the people, especially in the northern part of the country, were probably Christians. They were sharply divided by doctrinal differences into Monophysites, linked to the Jacobite church of Syria, and Nestorians. The Nestorians were the most widespread and were tolerated by the Sāsānian kings because of their opposition to the Christians of the Roman Empire, who regarded the Nestorians as heretics. The Monophysites were regarded with more suspicion and were occasionally persecuted, but both groups were able to maintain an ecclesiastical hierarchy, and the Nestorians had an important intellectual centre at Nisibis. By that time the area around the ancient city of Babylon had a large population of Jews, both descendants of the exiles of Old Testament times and local converts. In addition, in the southern half of the country, there were numerous adherents of the old Babylonian paganism, as well as Mandaeans and Gnostics.
In the early 7th century, the stability and prosperity of this multicultural society were threatened by invasion. In 602 Khosrow II launched the last great Persian invasion of the Byzantine Empire. At first he was spectacularly successful; Syria and Egypt fell, and Constantinople (modern Istanbul) itself was threatened. Later the tide began to turn, and in 627–628 the Byzantines, under the leadership of the emperor Heraclius, invaded Iraq and sacked the imperial capital at Ctesiphon. The invaders did not remain, but Khosrow was discredited, deposed, and executed. There followed a period of infighting among generals and members of the royal family that left the country without clear leadership. The chaos had also damaged irrigation systems, and it was probably at this time that large areas in the south of the country reverted to marshlands, most of which remained until modern times. It was with this devastated land that the earliest Muslim raiders came into contact. (See also Islamic world: Conversion and crystallization [634–870].)