It is certain that the first center of Shi‘ism had been the city of Medina and the pioneering Shi‘ah among the companions {sahabah} of the Prophet (S) lived in that city. During the reign of the first three caliphs, the Shi‘ah sahabah were scattered in the different cities and regions, with some of them holding political and military positions.
In this regard, ‘Allamah Muhammad Jawad Mughniyyah writes: The Shi‘ah sahabah played a pivotal role in the propagation and spread of Shi‘ism. Wherever they went, they were calling on the people toward Shi‘ism within the framework of the Qur’an and the Sunnah and through patience and fortitude, and on account of their companionship with the Prophet (S), they were held in high esteem and regard by the people, and their speech had tremendous impact.
Even the place like Jabal ‘Amil (in present day Lebanon) which was part of Sham, Mu‘awiyah’s sphere of influence, would become one of the most important centers of Shi‘ism by the grace of the presence of the great companion of the Prophet (S), Abu Dharr (al-Ghiffari).
During the latter part of the caliphate of ‘Uthman ibn ‘Affan, many Shi‘ah were living in the Muslim territories such that the name of ‘Ali (‘a) was always mentioned for caliphate. For this reason, ‘Uthman used to send requests to ‘Ali (‘a) during rebel gatherings in Madina, asking him to stay out of Medina for sometime and to go to his farm in Yanbu‘ with the hope that the rebels would be motivated less.
There were many Shi‘ah in Iraq especially during the time of ‘Uthman. For example, when the Shi‘ah of Basrah—in spite of the fact that the city was under the occupation and influenced by the Jamal {camel} Army propaganda—heard the news that the Commander of the Faithful ‘Ali (‘a) along with Muhajirun and Ansar was heading toward them, three thousand men only from the tribe of Rabi‘ah joined the Imam in Dhiqar. Their decision to join ‘Ali (‘a) was ideologically motivated, regarding the Imam as the caliph appointed by the Prophet (S) just as Baladhuri has this to say: “Among the Shi‘ah of ‘Ali were those from the tribe of Rabi‘ah.”
After ‘Ali (‘a) himself took control of the helm of government and went to Iraq, there was an extraordinary impact upon the spread of Shi‘ism in the region. Similarly, the rulers and governors of the Imam, most of whom were appointed from among the Shi‘ah, had remarkable contribution in the spread of Shi‘ism in other places. As Sayyid Muhsin Amin says, “Wherever the trustees of ‘Ali (‘a) went, the people there would become Shi‘ah.”
Of course, during that period, in addition to Sham which was totally under the influence of the Umayyads, other regions, such as Basrah and north of Iraq, also had inclinations toward ‘Uthman. The people in those places were inclined toward ‘Uthman on account of the settlement of ‘Uthman’s relatives there, and this inclination in the north of Iraq continued till the end of the second century AH.
Mecca also had anti-Hashimi and anti-‘Alawi tendencies since the Age of Ignorance {yamw al-jahiliyyah}. Similarly, both during the Age of Ignorance and the Islamic period, Ta’if was the same as Mecca. The other tribes of Quraysh were always competing with Banu Hashim and not willing to accept the leadership of Banu Hashim, and this is one of the reasons for Quraysh’s opposition to the Holy Prophet (S). The people of Ta’if, too, like that of Mecca, rejected the Prophet’s (S) invitation although they submitted after Islam’s acquisition of power.
From the time of Hajjaj, Shi‘ism was extended beyond the realm of Iraq and Hijaz. During that period, as a result of the extreme pressure and harshness of Hajjaj, Shi‘ah went out of Iraq and settled in other Muslim territories. Especially in the eastern part of the Muslim lands at the end of the first century AH, the Shi‘ah centers in Iran were gradually formed. In Khurasan, the ‘Abbasids took advantage of the people’s love for the descendants of the Prophet (S) and through the slogan, “the appointed one from among the progeny of Prophet Muhammad (S),” they gathered the people around themselves and utilized them in the struggle against the Umayyads.
The scattering of the Shi‘ah during the ‘Abbasid period was very obvious. In the east, in addition to Iran, the Shi‘ah went to Central Asia, India and the Caucasus, among others, and with the collapse of the Umayyad dynasty, the Shi‘ah were also able to exert influence in the west, especially in Africa where a Shi‘ah government of the Idrisis was established during the second century AH. Although their government was a Zaydi one, it can be regarded as a ground for the efforts of the Shi‘ah. Of course, their contact with the capital (Baghdad) and Medina had been less due to the existence of the Aghlabi government in Egypt which was formed to counter them.
In this manner, Shi‘ism during the second century AH was spread in both the eastern and western parts of the Muslim world, and in addition to Khuzestan, the mountainous region {jabal} (the regions around the Zagros mountain ranges) and central Iran, Shi‘ism was also spread in far-flung regions such as Central Asia, present day Afghanistan, Azerbaijan, Maghrib (Morocco), India, and Tabaristan.

Summary
The first center of Shi‘ism is Medina and the pioneering Shi‘ah used to live in this city. During the period of the first three caliphs, the Shi‘ah sahabah (companions of the Prophet (S)) was scattered in the different cities and regions and calling people toward Shi‘ism on the basis of the Qur’an. The transfer of ‘Ali (‘a) to Iraq had a tremendous impact on Shi‘ism in Iraq.
From the time of Hajjaj, Shi‘ism was extended beyond the realm of Iraq and Hijaz. During the ‘Abbasid period, the Shi‘ah in the east, in addition to Iran, also found their way into Central Asia, India, and the Caucasus, and in the west also, with the establishment of the Idrisi government in Maghrib, a fertile ground was paved for the influence of Shi‘ism.
The Shi‘ah-populated regions during the first century AH were confined to Hijaz and Iraq. Owing to the residence of the pure Imams (‘a) and Banu Hashim there, Medina was the first city where the Shi‘ah gathered together.
The second Shi‘ah-concentrated region next to Medina was Yemen because the people embraced Islam through ‘Ali’s (‘a) hand.