Jawaharlal Nehru wrote in The Discovery of India that “among the many people and races who have come in contact with Indians and influenced India’s life and culture, the oldest and most persistent have been the Iranians.” It is the kind of historical fact readily verified by ordinary experience. My grandfather was more fluent in Persian than in any other language; I grew up using Persian words in everyday conversations, eating food that originated in Persia, and listening to music whose most widespread and enduring forms—qawwali and the ghazal—were refined by a medieval poet in Persian. For nearly a millennium, Persian was the lingua franca of Asia: the language widely used by political and intellectual mandarins and necessary, too, for travelers such as Marco Polo and Ibn Battuta, who both deployed the language in China. Indeed, if Persian nationalism has maintained a profound sense of historical continuity transcending many different political regimes, it is because of its roots in the achievements of an expansive and long-lasting Persian civilization, or ecumene. Translated into many vernacular languages, …