But if you know anything about the history of Islam, you know how stunning a Shi'a takeover of Baghdad is, not that it's the first. As I write in my book:
The Sunni ‘Abbasids founded Baghdad in 762 on the banks of the Tigris River in the heart of ancient Mesopotamia after overthrowing their cousins the Umayyads (also Sunni) in the year 750 at the Battle of the River Zab. (The Umayyads did continue to rule in al-Andalus.) The first ‘Abbasid caliph, Abū al-ʿAbbās as-Saffāḥ (r. 750-754), from whom the ‘Abbasid dynasty derived its name, was the youngest uncle of the Prophet Muhammad. His successor, Jaafar al-Mansour (r. 754-775), founded Baghdad, and it was under al-Mansour’s rule that Islamic scholarship began to flourish.
The Umayyads and ‘Abbasids hailed, like the Prophet Muhammad, from the powerful Quraysh tribe from Mecca, 1,200 miles to the south. From its placement on the Tigris, though, Baghdad could hardly help but absorb the Persian culture that had existed there for centuries. The new city was founded only 20 miles from Ctesiphon, one of the ancient world’s great cities, and the capital of two important Persian empires, the Parthians and the Sassanids.
Baghdad flourished, both economically and intellectually, for roughly a couple of hundred years, but the Arab Sunni Caliph lost his grip on power as early as the 900s.
By 945, they had ceded control to a Shi’a Persian people called the Buyids, who traveled nearly 600 miles from the mountainous lands just south of the Caspian Sea to invade Baghdad, which they ruled over for about 100 years. The Buyids instituted Shi’a observances, such as the public mourning of Husayn, in the old Sunni capital. They often confined the Sunni caliph to his palace. And once, they put up posters on the outside of Sunni mosques insulting the Rashidun (the first four “rightly guided” Sunni caliphs), and Muhammad’s wife, ‘A’ishah, causing riots. The civil war that has broken out between Sunni and Shi’a since the U.S. invaded Iraq in 2003 that we see every day on the news is in some ways the most recent manifestation of these hostilities.
In Rolling Stone, Aikans writes that "The ministry's walls are emblazoned with iconography and slogans paying tribute to the Shiite martyr Imam Hussain, as are police stations and vehicles throughout the capital." (Husayn and Hussain are different spellings but the same man.)
Of course, the Sunni would overthrow the Shi'a. However, not the Arab Sunni or the original caliphate, but the Turks who had been streaming out of Central Asia for centuries and who the Arabs had used as slave soldiers. But that is another story.