The Shiites felt that Ali should have been the first of the caliphs.

NPR: Early Divisions at Root of Sunni-Shia Conflict

Since the time of the caliphs, however, Islamic law has been interpreted to forbid non-Muslims from entering these holy cities.

CENTERFORSECURITYPOLICY: Warrior of ideas

The first three caliphs of Sunni Islam, the pope-like figures for the Sunnis, are not viewed in high regard by most Shiites.

NPR: Early Divisions at Root of Sunni-Shia Conflict

Not without reason: although some Shia dynasties, such as the Fatimids in Egypt, did hold sway for long periods, Islam's early caliphs often oppressed their Shia subjects.

ECONOMIST: Shia Muslims

Quseir Amra is a well-preserved desert castle which acted as both a fortress with a garrison and a residence of the Umayyad caliphs, built in the early 8th century.

UNESCO: Office in Amman

While it is often depicted by its promoters as Koranic in character, in fact, it is largely man-made, the product of dictates and rulings by caliphs and scholars over many centuries.

CENTERFORSECURITYPOLICY: Shariah's Trojan horse

Also in India, Shiite and Sunni Muslims coexist without regular violence for the most part, although, as I said, in these ritual moments where the Shiites curse the caliphs, that can cause occasional rioting.

NPR: Early Divisions at Root of Sunni-Shia Conflict

The first section begins with Arab conquests following the death of the Prophet Muhammad in 632 and takes us to the year 1000, by which time Arab caliphs held sway over an empire stretching from Afghanistan to Spain.

WSJ: Islamic Art at the Louvre | By Lee Lawrence