Before the immense changes of the 2011 "Arab Spring," it was Sunni-Shia sectarian rivalry that preoccupied most political analyzes of the Middle East. The growing tensions and occasional clashes between believers in the two main strands of Islam have been major concerns. Upheavals within the Shia sphere of influence had altered the relationship: the Iranian revolution of 1979 changed the politics of Iranian Shiism, and impacted on Shia communities regionally, while the 2003 Anglo-American invasion of Iraq initiated a new phase of tension in Sunni-Shia relations. The specter of a sectarian war...