The expansion of Islam in the seventh century through both Mesopotamia and Persia did not eliminate the distinction of the two regions. Though politically unified under the Caliphate, this unity evaporated when the Caliphate fragmented. Moreover, the Shia branch of Islam, though a minority in most of the Islamic world, gradually became the majority in Iran. In the 16th century, Iranian Shiism was institutionalized under the Safavi shahs, while at about the same time Mesopotamia was brought under the rule of the Sunni Ottoman sultans.

The modern era in this region may be regarded as beginning with the dissolution of the Ottoman Empire after World War I. What at that time was still called Mesopotamia was apportioned to Britain. Iraq became quasi-independent in 1921, but British influence remained dominant, as it was in Iran. However, the negotiation framework of the specific boundary dispute along the Shatt-al-Arab goes back a few years previously, to the Constantinople Protocol of 1913, negotiated between the Ottoman Empire, Iran, Russia, and Britain, had defined navigation right in the Shatt-al-Arab in a way that ceded several islands to (British-influenced) Iran.

The Protocol also specified that the thalweg -- the center

In 1975, when the two parties came close to war over this issue, Iran accepted rather minor concessions rather than take the risk of war. In contrast, in 1980, Iraq was willing to risk war after failing to reverse these concessions. It is probable that the Iraqis never in fact expected the Iranians to concede, and were seeking a pretext rather than a favorable negotiated settlement.

Iraq also faced in Khomeini an ideologically much more dangerous regime than it had faced in the Shah. This was true both in general terms and in specific terms relating to the Shia population of southern Iraq. The militant Shia government of Khomeini had far more powerful levers of subversion than had been available to the Shah: the lever of religious sentiment. More generally, the messianic religious agenda of the clerics threatened to upend the entire Islamic world in a way that the secular government of the Shah had neither means or motive to do. This posed a long-term threat while at the same time presenting a short-term opportunity. The Baathist regime of Iraq had had poor relations with the conservative monarchies of the Gulf states, such as Saudi Arabia. Revolutionary Iran, however, posed a far greater threat to these monarchies, which had substantial Shia populations, and whose fundamental legitimacy the Khomeini government challenged and denied.

Pelletiere, Stephen C. (1992). The Iran-Iraq War: Chaos in a Vacuum. New York: Praeger.

This outcome practically upends the ne