The Sunni-Shia divide

The story of Islam's division between Sunni and Shia started with the Prophet Mohammed's death in 632.

There was a power struggle over who would succeed him in ruling the Islamic Caliphate, with most Muslims wanting to elect the next leader but some arguing that power should go by divine birthright to Mohammed's son-in-law, Ali.

That pro-Ali faction was known as the "Partisans of Ali," or "Shi'atu Ali" in Arabic, hence "Shia."

Ali's eventual ascension to the throne sparked a civil war, which he and his partisans lost.

The Shia held on to the idea that Ali was the rightful successor, and grew into an entirely separate branch of Islam.

Today about 10 to 15 percent of Muslims worldwide are Shia — they are the majority group in Iran and Iraq only — while most Muslims are Sunni.

"Sunni" roughly means "tradition."

Today, that religious division is again a political one as well: it's a struggle for regional influence between Shia political powers, led by Iran, versus Sunni political powers, led by Saudi Arabia.

This struggle looks an awful lot like a regional cold war, with proxy battles in Syria and elsewhere.