There were three Persian Empires before the rise of Islam.
- The Achaemenid Empire, 550–330 BC. This is the one Bible students often think of as the Persian Empire. This is the one founded by Cyrus the Great and that was eventually destroyed by Alexander.
- The Parthian Empire (or Arsacid Empire), 247 BC – AD 245.
- The Sassanid Empire, AD 224–651.
The Parthian Empire was not as strong as the other two, but the Sassanid Empire, the last empire of Iran, was a terror to the Roman Empire. I bring all this up because there is a good overview of the Sassanid Empire at the ASOR blog here, written by Michael R. J. Bonner.
Far from being weak and loosely organised, the Sasanian army matched the Romans blow for blow, and several Roman emperors died or were taken captive in battle with Iran. Famously, the 3rd-century emperor Valerian, was supposedly skinned, taxidermied, and put on display in the Persian capital in order to intimidate Roman embassies.
Long before the Roman adoption of Christianity, Sasanian Iran was the world’s first major confessional state. Ardashir I was supposedly descended from a line a Zoroastrian priests, and he and his family made Zoroastrianism the ideology of state. The Sasanian Empire had an uneasy relationship with other religions at first. Fear that Iranian Christians might side with Rome provoked persecutions. But in the early 4th century, the Persian king Yazdgard I granted toleration to his Christian subjects, and was even rumoured (falsely) to have converted. Other kings were not always as generous, but Iranian Christians became more and more numerous, and by the end of the 6th century they were holding high office at the court of king Khusro II.
Bonner also tells about how, just at the moment of the empire’s greatest strength under its great king Khusro II, the king was assassinated in a plot directed by his own son, and the empire was left susceptible to a new threat.