Iran's Revolutionary Guards are one of the most important forces in the Middle East today. As the appointed defender of Iran's revolution, the Guards have evolved into a pillar of the Islamic Republic and the spearhead of its influence. Their sway has spread across the Middle East, where the Guards have overseen loyalist support to Bashar al-Assad in Syria and been a staunch backer in Iraq's war against ISIS. In spite of their prominence, the Guards remain poorly understood to outside observers. In Vanguard of the Imam, Afshon Ostovar has written the first comprehensive history of the organization. Situating the rise of the Guards in the larger contexts of Shiite Islam, modern Iranian history, and international politics, Ostovar account of the Guards' history since 1979 demystifies the organization. Politics, power, and religion collide in this story, wherein the Revolutionary Guards transform from a rag-tag militia established in the midst of revolutionary upheaval into a military and covert force with a global reach. As he shows, the Guards have been fundamental to the success of the Islamic revolution, and their symbiotic relationship with the clerical leadership has been fundamental to the regime's ability to retain power. They have also exported Iran's revolution beyond its borders, establishing client armies in their image and extending Iran's strategic footprint in the process. Ostovar tenaciously documents the Guards' transformation into a power player and explores why the group matters now more than ever in both the region and the wider world.