The U.S. has accused Iran of a bizarre plot to assassinate Saudi Arabia’s American ambassador—a scheme that involved pressing Mexican drug cartels to make the kill in Washington. If the charges are true, the U.S. may have a bigger mess on its hands than it realizes, warns Shashank Joshi. Iran and Saudi Arabia have deep, hostile religious divides going back decades that occasionally spill over into international political drama. The Arab Spring has sharpened the tension, as Iran fears its geopolitical position is slipping. But though Iran has wreaked death and destruction on the region for decades, the West shouldn’t excuse Saudi Arabia’s imperialism and furtive stoking of sectarian anger.