The shadows of the late twentieth century are long, and for those who study the rise and fall of empires, they increasingly resemble the political atmosphere now surrounding Tehran. In the autumn of 1989, the Soviet Union still appeared monolithic. It commanded a nuclear arsenal capable of annihilating continents, presided over a sprawling security state, and claimed an ideological mission that had defined the century. Yet beneath the façade, the structural foundations were crumbling....