lands to the west of the Indus an Oxus Rivers. Conflict on the western borders began with the famous Greco-Persian Wars which continued through the first half of the 5th century BC and ended with the Persian withdrawal from all of their European territories. The empire had a centralised, bureaucratic administration under the Emperor and a large professional army and civil services, inspiring similar developments in later empires.
In 334 BC, Alexander the Great invaded the Achaemenid Empire, defeating the last Achaemenid Emperor Darius III at the Battle of Issus in 333 BC. Followed the premature death of Alexander the Great Iran came under the control of Hellenistic Seleucid Empire. In the middle of the 2nd century BC Parthia rose to become the main power in Iran and continued as a feudal monarchy for nearly five centuries until 224 AD, when it was succeeded by the Sassanid Empire. The Sassanids established an empire roughly within the frontiers achieved by the Achaemenids, with the capital at Ctesiphon. Most of the period of the Parthian and Sassanid Empires were overshadowed by the Roman-Persian Wars, which raged on their western borders for over 700 years. These wars exhausted both Romans and Sassanids,which arguably led to the defeat of both at the hands of the invading Muslim Arabs.
Middle Ages (652–1501)
The prolonged Roman-Persian wars, as well as social conflict within the Empire opened the way for an Islamic invasion of Iran in the 7th century. Initially defeated by the Rashidun Caliphate, Iran later came under the rule of their successors the Ummayad and Abbasid Caliphates. The process of conversion of Iranians to Islam which followed was a prolonged and gradual process. Under the new Arab elite of the Rashidun and later Ummayad Caliphates Iranians, both Muslim (mawali) and non-Muslim (Dhimmi), were discriminated, being excluded from government and military, and having to pay a special tax. In 750 the Abbasids succeeded in
