Thousands of Shiite pilgrims pray near the holy shrine of Imam Hussein, to mark the anniversary of the birth of Shiite Imam Al-Mehdi al-Muntadhar, in Karbala. (AP Photo/Alaa Al-Marjani)

It is in Iraq that Shiism’s major defeats took place in the seventh, eighth and ninth centuries and, hence, their greatest shrines are here. For that reason they also are the most important declared targets of the Sunni jihadists now making their way across Iraq.

Ali, the Prophet’s son-in-law and the first Shia Imam, was attacked and killed while at prayer and is buried in Najaf. His son Hussein, the Third Imam, was killed in the Battle of Karbala and is buried there.

(To this day, many Shiites carry a small clay tablet made from the soil of Karbala on which Hussein’s blood was shed. They put the tablet on the ground wherever they pray and press their foreheads to it. Sunnis have no such practices in their worship.)

Samarra is the site of another great Shia shrine, the al-Askari Mosque, where Shiism’s last two visible imams are buried. Since the population around the mosque now is mostly Sunni, Samarra has become a heavily contested battleground in the current conflict.