For all the sectarian differences between Saudi Arabia and Iran, what divides the two countries most may be the thing they have in common. Both regimes have predicated their legitimacy on a transnational mission of exporting religion and safeguarding Islam. Following the Arab awakenings and the collapse of the regional state system that followed, their competition for power has only become more urgent. For months, the Islamic republic had been warning the Saudis not to harm the dissident Shia cleric Sheikh Nimr al-Nimr. The kingdom’s rash decision to kill him probably stems from a sense of vulnerability. Weakening oil prices are sapping the Sauds’ petroleum wealth just as the jihadis of Isis challenge the Islamic orthodoxy that underpins their claim to power.