In this succinct, probing survey of a major divide in the Muslim world, Louër explores relations between the Shiites and the Sunnis in seven different countries in the Middle East and South Asia. She does not tap new sources or make many new interpretations, but she compellingly mingles analysis of Shiite and Sunni doctrine and an examination of the political dynamics between the sects. Neither camp fully accepts the legitimacy of the other—although coexistence and cooperation have occurred, as in Mughal India. A major watershed was the advent of the Safavid dynasty in Persia in the sixteenth century, which wed Shiism to a geopolitical entity wedged between the Ottoman and Mughal empires. Ever since, the rivalry has become as much geopolitical as doctrinal and is more prone to militant and violent forms of confrontation (as exemplified by the evolution of Yemen’s Houthis, a Zaydi Shiite group).