The Sassanid Empire (226-651 A.D.) was a monarchy in what is now Iran. It was the only nation to use Zoroastrianism as the state religion. Fought a number of wars with the Byzantine Empire due to their persecution of Christians. The empire was destroyed in 651 by the first of the Islamic Caliphates after a difficult war with the Byzantines had left them unable to affectively defend themselves.
The Sassanid Empire was founded in 226 A.D. by Ardashir I. He considered himself to rule a re-emergence of the Persian empire. At the height of power, the empire extended to the borders shown in the image. The most famous ruler of the Sassanid Empire is Khosro I.