Shia politics roared to the center of power in Iraq after the U.S. invasion in 2003 after decades of marginalization under Saddam Hussein. A new generation of clerics and politicians, self-described as Shia Islamists, promised a revolution in law and governance. The meaning of Shia Islamism today is murkier than ever. A confusing array of factions all wear the label, which does nothing to explain...