Most Americans are vaguely aware of a few rulers of ancient Achaemenid Persia: Cyrus, Darius and Xerxes, whether from the Bible, from historically grounded films like 300, or in the rare case, from reading Herodotus' The Histories. More recently, Iran has loomed large due to its geopolitical significance, and for Americans of a certain age, the Shah Reza Pahlavi and his successor Ayatollah Khomeini loom large as figures who for a time monopolized television screens and front pages of news magazines. But these are drops in a bucket; the history of Persia or Iran, the two being synonymous, spans nearly 3,000 years. The Farsi language in modern Iran is directly descended from Old Persian, the language of Cyrus the Great, the Persian Empire's founder.

Directly in the middle of Persia's millennia of history are the Sassanids, who ruled Iran for four centuries after the fall of the Parthians and made appearances in Roman histories, playing an instrumental role in the deaths of Emperors Valerian and Julian. Most Westerners will know the Sassanids only as the name of the last pre-Islamic dynasty of Iran, the last guardians of Zoroastrian Iran, fated to be washed away by history. But according to Michael Bonner's The Last Empire of Iran, they served as a critical prelude to the emergence of Islamic and modern Iran.

In this episode of Unsupervised Learning, Razib discusses with Bonner the role of religion and ethnicity in Sassanid Iran, how they relate to what came after during the Islamic period, and how they shape Iran's civilization today. Bonner also covers the role of the Sassanids in the "world war" of Late Antiquity between Rome and Persia, as the last great Shah of the Sassanids almost completed the conquest left unfinished by the Achaemenids. Though the Sassanids and their civilization were overthrown by Islam, Razib and Bonner entertain the idea that Islam, as it developed after the Abbassid Caliphate in the 9th century AD, cannot be understood without the key of Zoroastrian Persian influence.