The term Persia has been used for centuries, chiefly in the West, to designate a region of southern Iran formerly known as Persis or Parsa; the name of the Indo-European nomadic people who migrated into the region about 1000 BC, eventually supplanting the Assyrians and Chaldeans. The first mention of the Parsa occurs in the annals of Shalmanesar III, an Assyrian king, in 844 BC. Cyrus II (559-529 BC), also known as Cyrus the Great, was heir to a long line of ruling chiefs in Mesopotamia and was the founder of the Persian Empire; he was called the father of his people by the ancient Persians. In 550 BC, Cyrus, the Prince of Persia, revolted against the Median King Astyages and welded the Persians and Medes together into one powerful force. Cyrus consolidated his rule on the Iranian Plateau and then extended it westward across Asia Minor. In October 539 BC, Babylon, the greatest city of the ancient world, fell to his Persian forces. Cyrus also oversaw the construction of a series of great roads to link together the territories that he had conquered. Although Cyrus was a great military conqueror, he was also a fair ruler; he allowed the Jews to return from Babylon to their homeland in Palestine. His dynasty, known as the Achaemenids, ruled Persia for two centuries.

Following the death of Cyrus' heir, Darius I (522-486 BC), a leading general and one of the princes of the Achaemenid family, proclaimed himself king following the suppression of a number of provincial rebellions and challenges from other pretenders to the throne. Darius was in the mold of Cyrus the Great - a powerful personality and a dynamic ruler. To consolidate his accession, Darius I founded his new capital of Parsa, known to the Greeks as Persepolis ("Persian City") and expanded the ranks of his personal bodyguard, the Immortals. The elite force drew its name from the fact that no matter how many men were lost, the Persian Emperor would always pay the cost to restore the Immortals back to their original strength. Although Darius consolidated and added to the conquests of his predecessors, it was as an administrator that he made his greatest contribution to Persian history. During his reign, political and legal reforms revitalized the provinces and ambitious projects were undertaken to promote imperial trade and commerce; coinage, weights and measures were standardized, and new land and sea routes explored and established.

Such activities, however, did not prevent Darius from following an active expansionist policy. Campaigns in the east confirmed gains made by Cyrus the Great and added large sections of the northern Indian subcontinent to the list of Persian-controlled provinces. Expansion to the west began about 516 BC when Darius moved against the Greek colonies along the coast of Asia Minor. Xerxes (486-465 BC), son and successor of Darius I, was determined to continue the Persian conquest of the west and is best known for his massive invasion of Greece from across the Hellespont in 480 BC, a campaign marked by the battles of Thermopylae, Salamis, and Plataea. Although successful in the pacification of Egypt and suppression of a Babylon revolt, his defeat by the allied Greek city-states spelled the beginning of the decline of the Persian Empire. In Xerxes' last years, he squandered the once-enormous treasury he had gathered through trade and taxation by launching vast construction programs, most of which never finished.

The death of Xerxes was the final turning point in Persian influence. Occasional flashes of vigor and ability by some of Xerxes' successors were too infrequent to prevent eventual collapse. The final act was played out during the reign of Darius III (336-330 BC), who was defeated at the Battle of Granicus (334 BC) by Alexander the Great. Persepolis fell to the young Macedonian conqueror in April 330 BC, and Darius, the last Achaemenid, was murdered in the summer of the same year while fleeing the Greek forces. In the struggle for power after Alexander's death, Seleucus I brought under his control the Persian provinces of Alexander's empire. But this unity was short-lived, as the Indian holdings successfully revolted and the Seleucid kingdom broke into the competing nations of Parthia and Bactria. Parts of the Seleucid kingdom lasted for two centuries, but it was eventually swept aside by the Parthians, who founded an empire that stretched almost as far as Persia under the Achaemenids. The Romans and Parthians struggled against one another for centuries over control of Mesopotamia, with the Parthians usually holding onto most of the Fertile Crescent. But in 224 AD the Parthians were themselves overthrown by a new Sassanid dynasty that revived many of the customs of the Achaemenids, such as the Zoroastrian religion. The Sassanids fought a series of debilitating wars with the Byzantine Empire in the 6th and 7th centuries, which fatally weakened the Persian Empire when the Arabs exploded onto the scene. In a series of decisive battles between 633 and 642, the Arabs conquered and destroyed the Persian Empire; since this time, Persia (modern Iran) has largely belonged to the Arab world. The customs and religion of ancient Persia were destroyed and the population absorbed into the surrounding Islamic culture; only a few remnants survive today.