Persia (Iran) was a great empire in the ancient world. Its king was called Shahinshah or the king of the kings. It was conquered by the Greeks under Alexander the great about 327 BC.
Around 150 AD, a new empire came into being in Persia. It lasted for nearly five centuries before it was conquered by the Arabs in the days of Hazrat Umar (ra). The most famous Persian emperor was Nowsherwan. His title was Khusrow (Kisra in Arabic and Cosroe in Latin). He fought many battles against the Justinian and recovered the Persia territories captured by the Byzantines. Nowsherwan was also famous for his love of justice, and was popularly known as Nowsherwan the just. The religion of the Persian Emperors was Zoroastrianism.
