Book excerpt: In early 2019, the Islamic Republic of Iran marked its fortieth anniversary, despite decades of isolation, political pressure, sanctions and war. Observers of its security policies continue to try and make sense of this unlikely endurance. Some view the regime as a purely rational actor, whose national security decisions and military affairs are shaped by the same considerations as in other states. Others believe that it is ideology driving Tehran's strategy. Either way, virtually everyone agrees that the mullahs' policies are fundamentally different from those pursued by their monarchical predecessors. No Conquest, No Defeat offers a historically grounded overview of Iranian national security. Tabatabai argues that the Islamic Republic is neither completely rational nor purely ideological. Rather, its national security policy today is largely shaped by its strategic culture, a product of the country's historical experiences of war and peace. As a result, Iranian strategic thinking is perhaps best characterized by its dynamic yet resilient nature, one that is continually evolving. As the Islamic Republic enters its fifth decade, this book sheds new light on Iran's controversial nuclear and missile programs and its involvement in Afghanistan, Iraq, Lebanon, Syria and Yemen.