From its outset, Shiʿi Islam has been associated with Iraq. In 661 ʿAli ibn Abi Talib was assassinated in a mosque in Kufa. ʿAli's son Husayn, who laid claim to the caliphate, was killed with his companions in a battle that took place in the plain of Karbala in 680. Many of the twelve Shiʿi imams spent at least part of their lives there. The four most sacred Shiʿi shrine cities—Najaf, Karbala, Kazimayn, and Samarra—are in Iraq. Since the early stages of Islamic history, much of Shiʿi learning was centered in Hilla, Baghdad, Najaf, and Karbala. And, finally, Iraq was also once a territory ruled by Shiʿi dynasties, notably the Buyids (945–1055).
But all this by itself cannot explain how the Shiʿs came to form the majority of the population of modern Iraq. Although conversion from Sunnism to Shiʿism took place in Iraq throughout Shiʿi history, it was confined mainly to cities, where only a small fraction of the population lived. Occasionally, some Arab tribes were also converted to Shiʿism—for example, the Bani Sulama, the Tayy, and the Sudan in the marshes near Khuzistan during the Mushaʿshaʿ Arab Shiūi dynasty of the 15th to 16th centuries. Nonetheless, there is no evidence that would suggest that the Shiʿis were ever close to forming a majority of Iraq's population before the 19th or even the 20th century.