After ISIS struck two Shi’a mosques in Saudi Arabia with suicide bombs, the worst sectarian violence the Kingdom has seen in recent times, Saudi authorities have begun a security crackdown in the country’s Eastern Province. But that hasn’t reassured the country’s minority Shi’a population, who are now forming militias for protection. In turn, the authorities are alarmed by these volunteers, and report that some are already being arrested. In the midst of the Saudi war on the Shi’a Houthi in Yemen, anti-Shi’a sentiment within the Kingdom is on the rise. ISIS is making use of that feeling to recruit; as one expert put it, “Rather than going after foreigners in well-defended compounds, [young Saudis] are blowing up fellow Saudis, who happen to be Shia.”When ISIS struck Saudi Arabia two weeks ago, we noted that, “An attack like this both strengthens ISIS’ claim to be a pan-Sunni, anti-Shi’a “defense” force and sows division within what ISIS would see as a rival for the leadership of the Sunni world, Saudi Arabia.” The development of the Shi’a militias would appear to show the early success of that strategy. Divide et impera is a universal principle, after all—and ISIS has thrived since its beginning on ethnic strife.The dark hopes of groups like ISIS for a region-wide sectarian war have been having a good run lately, with local forces separating into religious camps from Syria to Yemen. In the absence of an American-guaranteed regional balance of power, such patterns are likely to continue. The President has been focused solely on securing an Iran nuclear deal for some time now, convinced that all the cards will fall the right way from there. But what if the sectarian strife continues afterward—or, as is likely, actually intensifies in the wake of Iranian sanctions relief?