Who Are Iraq’s Sunni Arabs and What Did We Do To Them? by Juan Cole [View all]
https://www.commondreams.org/view/2014/06/18-0

A young Iraqi girl waits outside of her house during a clearing operation in the Rasalkoor District of Mosul, Iraq, May 27. (Credit: Joint Combat Camera Center Iraq / cc / Senior Airman Kamaile Chan)
The two great branches of Islam coexist in Iraq across linguistic and ethnic groups. There are Sunni Arabs and Shiite Arabs, Sunni Kurds and (a tiny minority of) Shiite Kurds. Arabs are a linguistic group, speaking a Semitic language. Kurds speak and Indo-European language related to English.
Sunnism and Shiism as we know them have evolved over nearly a millennium and a half. But the difference between them begins after the death of the Prophet Muhammad in 632 AD (CE) in the city-state of Mecca in western Arabia. Muhammad, the son of Abdallah, had derived from the noble Quraysh clan. Those who became the Shiites insisted he should be succeeded by Ali, his cousin and son-in-law (and the next best thing to a living son). This dynastic principle was rejected by the group that became the Sunnis. They turned for leadership to prominent notables of the Quraysh, whom they saw as caliphs or vicars of the Prophet. The first three caliphs had given their daughters in marriage to the Prophet and so were his in-laws, but Sunni principles said that they neednt have been any prominent, pious male of the Quraysh would have done.
There is a vague analogy to the split between Catholicism and Protestantism, on the difference between seeing Peter as the foundation of the Church and of seeing Paul as that.
Iraq was part of the medieval caliphates the Orthodox Caliphs, then the Umayyad Arab kingdom, and then the Abbasids. In 1258 the invading Mongols (themselves Buddhists and animists) sacked Baghdad and executed the last caliph. It is said that they were warned that it was very bad luck to shed the blood of a caliph, so they rolled him up in a Persian rug and beat him to death with hammers.