List of ancient great powers
The Sassanid Empire is the name used for the fourth Iranian dynasty, and the second Persian Empire (226–651). The empire's territory encompassed all of today's Iran, Iraq, Armenia, Afghanistan, eastern parts of Turkey, and parts of Syria, Pakistan, and large parts of Caucasia, Central Asia and Arabia. During Khosrow II's rule in 590–628 Egypt, Jordan, Palestine, Lebanon were also briefly annexed to the Empire, as well as far west as western Asia Minor. The Sassanid era, encompassing the length of the Late Antiquity period, is considered to be one of the most important and influential historical periods in Iran. In many ways the Sassanid period witnessed the highest achievement of Persian civilization. The empire is furthermore known for being the arch-rival of the neighboring Roman–Byzantine Empire for a period of over 400 years. As the Parthians were replaced by the Sassanids, they carried on the already century long lasting Roman–Persian Wars, which would eventually become the longest conflict in human history.
Islam in Iran
The Islamic conquest of Persia (637–651) led to the end of the Sasanian Empire and the eventual decline of the Zoroastrian religion in Persia. However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic polity. Islam has been the official religion of Iran since then, except for a short duration after the Mongol raids and establishment of Ilkhanate. Iran became an Islamic republic after the Islamic Revolution of 1979.