[this chapter was especially illumantive to me regarding the question of Sunni-Shi'i distribution in old Iraq (before the 1800s) and what did "Iraq" exactly mean at the time]
Iraq may be said to have been, in the time of the monarchy, divided into three religious zones.
One of these zones, the most populous, was and remains the home of Shiism. it covers all the provinces to the south of Baghdad. In its ethnic composition it is Arab except for concentrations of Iranians in Basrah and the Holy Cities of Najaf and Karbala. Its Shiism is not unbroken. Here and there it is interspersed with islands of Sunnism, which are urban in character and, in their size, inconsiderable, except in Basrah and Nasiriyyah, where there are strong Sunni minorities, and in the town of Zubair, southwest of Basrah, which is entirely Sunni.
A second religious zone, embracing Arab-inhabited valleys of the Euphrates above Baghdad and of the Tigris between Baghdad and Mosul, is the domain of Sunnism. Here only small Shii minorities at Dujail, Balad, Samarra, and a string of Turkmen settlements, some of which are Shi'i as in Tal Afar and Toz Khomatu, breach the Sunni continuity.
The third religious zone conincides with the Kurdish rain-fed mountain crescent in the north and northeast of Iraq. This zone is also Sunni, but unlike the Sunni Arab region, it was in the period of monarchy permeated by mysticism and by its practitioners, the Sufis. This is not to say that there were no traces of Arab Sufism, however, in monarchic days had none ouf the outward vigor of Kurd mysticism.
EXPLANATIONS FOR THIS RELIGIOUS CONFIGURATION
Nearly at thousand years ago, Abu Bakr al-Khawarizmi (d 993) envied the people of Iraq because, as he put it, "in their midst are the tomb-sanctuaries of [Ali]...and of Hussein, and because Shiism is Iraqi." At the time the name Iraq referred not to the territory of present day Iraq, but only to that part which lay south of a line connecting Anbar (or according to another view, Haditha) on the Euphrates and Tikrit on the Tigris, that is, it coincided, except for Baghdad and areas north of Baghdad, with the abode of the Shi'is.the heart of the sect was then, as now, the Middle Euphrates, the role of the Buwayhids in Baghdad (from 945 t o1055) and al-Mazyad of Bani Asad from Hilla to Basra (1012 to 1150) helped or consolidated the advance of Shi'i principles. So did also the power that the Shii Arab dynasty of the Musha'sh' Saadah wielded from the outskirts of Baghdad to the Gulf in the middle of the fifteenth century. But before and after that time the country passed through a succession of conquests: the Euphrates and Tigris changed their main beds ; medieval towns, like Wasit and Madai'n disappeared, new towns, like Amara and Nasiriyah came into life, old tribes were scattered or subdued, and new tribes from Arabia moved into the river valleys, Yet in the midst of all the vicissitude and instability one feature persisted: the overwhelmingly Shi'i character of this zone. How can one account for this Shii continuity, particularly in the face of long centuries of apparent Sunni dominance, the dominance of the Ottoman Turks (1534-1622, 1638-1917) and their nominal vassals, the Georgian Mamluks (1749 - 1831)??
Apart from the power of persistence natural to religions and in particular to aggrieved sects, one obvious factor making for the perpetuation of Shi'i influence was :
 the presence of the Shii sanctuaries of Najaf and Karbala, and Shii schools at Najaf and Hillah.
 The commercial and religious intercourse that the Shias of Iraq maintained, if interruptedly, with Shi'i Persia.
 The contagion of the environment, Bedouin tribes who moved into the Shi'i zone - and Islam sat lightly on the Bedouins - tended in time, ti wold appear, to adapt themselves to its beliefs and practices.
 The missionary zeal of the mumans, who were itinerant men of religion.
It may be wondered how Shii conversions took place seemingly under the very nose of the Sunni government, the explanation is simple. During the greater part of the Ottoman period the write of the authorities ran precariously outside the main towns.
 The conversions may have even come about on account of the government: the tribes' intolerance of government - any government. and their association of government with oppression, plus the fact that the government was Sunni, may have eased the task of the mumans and the transition to Shiism.
The government accorded the Shi'is full liberty to make their devotions in their own manner in all the places that they considered sacred, apparently because it stood to gain from the flow of pilgrims to Iraq. But in all other places, as in Basrah or in Baghdad proper, they were denied the free exercise of their religion. This rule must have been relaxed in the course of the later part of the nineteenth century, under the monarchy the religious freedom of the Shiis became complete.
In turning to the Sunni Arab zone, the thing that catches attention is that Shiism never penetrated it in strength, while a Shii dynasty, the Hamdanis wielded authority in Mosul between 905 and 979, but it hardly made a dent in the Sunni loyalty of its inhabitants. An attempt to encourage Shiism by Badr-ud-Din Lulu, a slave who ruled Mosul for about forty years in the first half of the 19th century, failed to evoke any response. with minor exceptions, the whole region remained steadfast in its attachment to Sunnism down to our own time. Perhaps the most crucial explanation for this is the fact that, in their economic relationships, the regions of Mosul and the upper Euphrates were oriented toward Sunni Syria and, in a lesser degree, toward Sunni Turkey. Indeed, it would not be going too far to say that in the days of the monarchy the people of Mosul were closer in outlook and temperament to the Arabs of Syria, specficially of Aleppo, than to the Arabs of central and southern Iraq. (True, even the Mosuli accent, which I speak since I'm half-Mosuli, has more similarity with the softer Levantine dialects of Arabic than the harsher, more tribal accent of Iraq proper and the Gulf.)