Nearly every tension across the Middle East — from the war in Syria to Saudi Arabia’s blockade of Qatar to the United States backing out of the Iran nuclear deal — seems to hinge on the rising animosity between largely Shia Iran and Sunni Saudi Arabia, which is undergoing a generational shift with the rise of its new, young leader Mohammed bin Salman. What does it take to understand the long-running shadow war between the two powers — and where are they taking us next?