Iran’s Islamic Revolutionary Guard Corps was born in the first weeks of the Iranian Revolution, in February 1979, and has arguably become the Islamic Republic’s most powerful institution. It spearheaded all of Iran’s engagements during the Iran-Iraq War. The IRGC’s Quds Force, commanded by the redoubtable Qasem Soleimani, has internationalized the Iranian Revolution by involving itself in conflicts in Lebanon, Iraq, Syria, and Yemen. And the IRGC’s domestic feeder organization, the Basij militia, has been a bulwark of internal support for Iran’s supreme leader, Ali Khamenei. Ostovar provides a careful and dispassionate history of the organization and its domestic and foreign exploits. The IRGC cannot be reduced to a vanguard of impassioned religious warriors, although that aspect of it is important. It has economic interests to protect and is closely allied with Khamenei. Ostovar asks but does not answer the question of whether the next supreme leader will follow Khamenei’s example and ally himself closely with the IRGC or align himself more with the preferences of Iranian civil society. Whatever course he chooses, the IRGC will rely on its proven survival instincts. Donald Trump will be the seventh U.S. president it has confronted.