For four decades Saudi Arabia and Iran have vied for influence in the Muslim world. At the heart of this ongoing Cold War between Riyadh and Tehran lie the Sunni-Shia divide, and the two countries' intertwined histories. Saudis see this as a conflict between Sunni and Shia; Iran's ruling clerics view it as one between their own Islamic Republic and an illegitimate monarchy. This foundational schism has played out in a geopolitical competition...