The 6th century BC was witness to the establishment of these Persians in the present-day region of Fars. Fars (or Persis to the Greeks) was a recognizable district of the Assyrian Empire like the neighboring but greater Media. Persian rulers, claiming descent from one Achaemenes (or Hakhamanesh), took over the rule of Media from Astyages in the middle of the 6th century BC. In an amazingly short time Cyrus could extend his conquests from Elam and Media west and north. He pushed into Asia Minor and, upon defeating the Lydians, established the greatest Persian Empire, which was to endure long under his successors, the Achaemenians.
Cyrus made Ecbatana, the seat of Median Kingdom, his capital, while retaining his Persian capital at Susa and creating and embellishing his new residence at Pasargadae. Today the first lies buried under the modern city of Hamadan, but Pasargadae, 130 km to the northeast of Shiraz, remains one of the most evocative sites in the country.The dynamic new state was, however, disturbed almost from the start by dynastic troubles. Cambyses 11, son of Cyrus, did away with Smerdis, another son of Cyrus, in order to have unchallenged power, but when Cambyses was absent on a successful raid into Eg, an impostor claito be Smerdis appeared, and usurped the throne.
A civil war ensued, and after Cambyses died, a new claimant, Darius I, descended from another line of Achaemenians could carry out his claims, and after putting down disorders and suppressing all opposition, molded the administration of the empire into the centralized system that was remarkable for its efficiency. Darius was a dynamic personality who extended the empire to its farthest limits, in the course of which he first challenged the Greeks in a contest continued by his successors. The palatial precinct of Persepolis, which he erected on the lowest slope of Rahmat Mountain in mid-Fars near Shiraz, displays a magnificent image of imperial grandeur with its portrayal of the subject peoples bringing their tributes to the King. He founded a centralized system supported by an intricate and excellent system of communication. Thus, the Persians were the first important ancient people to use the horse efficiently for communication and transport. Darius also continued and broadened Cyrus's policy of encouraging the local cultures within the empire, allowing the people to worship their own gods and keep their own customs so long as their practices did not conflict with the necessities of Persian administration. Despite this tolerance there were rebellions by the Egyptians, and the neighboring Lydians and Babylonians, all of which were ruthlessly suppressed by Darius.

The religion of Persia itself was Zoroastrianism, and the unity of Persia may be attributed in part to the unifying effect of that broadly established faith. Darius was a patron of arts, as can be seen from the magnificent palaces standing on high terraces beautifying the capitals of Susa and Persepolis.
Darius was also a conqueror. Persian rule was pushed far eastward past the Arius (Hari Rud) river into modern Afghanistan and Pakistan. Egypt had already been attacked by Cambyses, and although it was to prove recalcitrant and rebellious, succeeding Persian Kings were to maintain hegemony there. Darius pushed as far north as the Danube in his exploits.
At the beginning of the 5tq", century BC, however, the Ionian cities were involyed in trouble with the great king. Darius put down the rebellion, then organized an expedition to punish the city-states in Greece proper that had lent aid to the rebellious cities. The expedition was the beginning of the Persian Wars. Ultimately Darius' army was defeated at Marathon, and his son Xerxes I, who succeeded to the throne in 486 BC, fared no better at Salamis.
The Greeks had successfully defied the power of the great king. The effects of the Greek victory were, however, confined to Greece itself and had no consequences in Persia. Nor did the Greek triumph exclude Persia from taking part in the affairs of the Greek world. Persian influence was strong, and Persian gold was poured out to aid one Greek city-state or another in the interminable struggle for power.
In the time of Artaxerxes the difficulties of maintaining so wide an empire began to appear. Some of the governors (satraps) showed ambitions to rule, and the Egyptians, helped by the Athenians, undertook a long rebellion. Violence against the great King himself was a disturbing factor.
The most celebrated of the dynastic troubles occurred in the rebellion of Cyrus the Younger against Artaxerxes II, which came to an end with the death of Cyrus in the battle of Cunaxa (401 BC). Cyrus' defeat was recorded in Xenophon's Anabasis, and although the importance of Cyrus' revolt may be exaggerated it cannot be denied that there were signs of decay within the empire itself.
In 334 BC, Alexander the Great and his Macedonian army of about 40,000 men crossed the Hellespont and routed the Persians on the Granicus. The battle of Issus followed in 333, and in 331 the battle of Gaugamela brought an end to the Achaemenian Empire. Darius II, last of the great Kings, fled east before the conqueror to the remote province of Bacteria, where he was assassinated by his own cousin, Bessus. Alexander also came east and, defeating Bessus, had the whole empire in his grasp. Before this, he had reached Persepolis, where as the climax of a drunken carouse, he burnt down the great palace of the king of kings. This he afterwards declared was the revenge of Greece for the burning of Athens by Xerxes. Ghirshman gives some convincing reasons (without coming to any definite conclusion) for thinking that Persepolis caught fire as a result of an accident.
Whatever the truth, it is a strange irony that there is still plenty to show for the past glories of Persepolis, while Susa, which Alexander preserved, is little but moldering mounds of earth.

During the Achaemenian period, Iran had managed to create one of the most advanced civilizations of the world. Paved roads were built for horse-drawn traffic from the shores of the Mediterranean to India. Rest houses and stables known as the caravansaries were built at distances not exceeding 30 km. The first courier service of the world was established in Iran to dispatch the mail throughout the vast Achaemenian Empire. A canal was built from the Red Sea to the Nile. Guards were posted along the roads. Travelers were searched and inspected. Exploitation of mines and development of agriculture were encouraged; chemistry, cloth weaving, embroidery, as well as carpet weaving were initiated; Iranians were accustomed to eating at table and sleeping on wooden beds.

Seleucids
Alexander went on to India and created the greatest empire the world had yet seen. It lasted, however, only for the brief period of his life and then was torn apart by the quarrels of his successors (the Diadochi).
Persia fell for the most part to Seleucus I, who appeared as the master of Alexander's Eastern Dominions and married an Iranian wife. The grasp of Alexander's successors (the Seleucids) on the vast territories of Iranian empire was weak administratively, although they did introduce a vital Hellenistic Culture, mingling Greek with Persian elements. The process was by no means one-sided. Large numbers of Greek civilians were settled in the cities founded along the northern, western and southern edges of the country -in Bacteria, at Hecatompylos (Damqan), Rhages (Rey), Kangavar and Nahavand in the Zagros. In and around these cities, Greeks and Iranians were fused by intermarriage, bilingualism, and a mingling of Greek oriental religious cults.
Yet the end came, not primarily because the Greeks succumbed to oriental influences or were overwhelmed by sheer numbers, but from external causes -the rise of the Roman Empire in the west, and the first of the many nomad invasions, that of the Parthians, in the east.