The Sunni branch of Islam is by far the largest, accounting for 85% or more of Muslims worldwide. Sunnis look not only to Muhammad’s family but the Prophet’s companions for understanding and wisdom.

Differences with Shia, the second-largest branch, have given rise to countless conflicts over the centuries. The schism has to do with rules of succession from the earliest years of Islam. Sunnism holds that the most learned and important leaders should decide who should lead. Shia holds that leadership can be inherited only by those with the bloodline of the prophet Muhammad.

Where temporal authority derives from religious authority, assigning religious authority is a matter of supreme temporal importance. As far as the two sects are concerned, only one can be right.

Iraq’s population is 55% to 65% Shia, but it was ruled for decades by a secular Sunni government of Ba’athist Arab nationalists until the American overthrow of Saddam Hussein. Iran, to the east, is at least 90%

Shia and aggressively seeks to expand its influence into majority-Shia Iraq. Iraq’s northwestern neighbor Syria is overwhelmingly Sunni but is ruled by a secular Shia Ba’ath party that bitterly opposed the Sunni Ba’athists in Iraq.