The February 2006 bombing of the Askariya Shrine in Samarra, a mosque linked closely to historical imams of Shiism, sparked an onslaught of Shia on Sunni violence. Dia Hamid/AFP/Getty Images hide caption

itoggle caption Dia Hamid/AFP/Getty Images

Demonstrators hold up a poster of exiled Shiite Muslim leader Ayatollah Khomeini during an anti-shah demonstration in Tehran, the Iranian capital, on Dec. 10, 1978. The Iranian revolution in 1979 had a powerful effect on the wider Muslim world, particularly among Shiites. Michel Lipchitz/AP hide caption

itoggle caption Michel Lipchitz/AP