The Achaemenid Empire was the successor state of the Median Empire, expanding to eventually rule over significant portions of the ancient world, which at around 500 BC stretched from the Indus Valley in the east, to Thrace and Macedon on the northeastern border of Greece and eventually controlling Egypt. It was unified by a complex network of roads, ruled by monarchs, to become the largest and greatest empire the world had yet seen. By the 5th century BC the Kings of Persia were either ruling over or had subordinated territories encompassing not just all of the Persian Plateau and all of the territories formerly held by the Assyrian Empire (Mesopotamia, the Levant, Cyprus and Egypt), but beyond this all of Anatolia and the Armenia, as well as the Southern Caucasus and parts of the North Caucasus, Azerbaijan, Uzbekistan, Tajikistan, all of Bulgaria, Paeonia, Thrace and Macedonia to the north and west, most of the Black Sea coastal regions, parts of Central Asia as far as the Aral Sea, the Oxus and Jaxartes to the north and north-east, the Hindu Kush and the western Indus basin (corresponding to modern Afghanistan and Pakistan) to the far east, parts of northern Arabia to the south, and parts of northern Libya to the south-west, and parts of Oman, China, and the UAE. In the 5th century BC, it is estimated that 50 million, or around 45% of the world population lived in the Achaemenid Empire, which would make it the largest empire in history in terms of percentage of world population.
The term Achaemenid is in fact the Latinized version of the Old Persian name Haxāmaniš (a bahuvrihi compound translating to “having a friend’s mind”. Achaemenes was himself a minor 7th-century ruler of the Anshan (Ansham or Anšān) located in southwestern Iran. It was not until the time of Cyrus the Great, a descendant of Achaemenes, that the Achaemenid Empire developed the prestige of an empire and set out to incorporate the existing empires of the ancient east. The historical mark of the Achaemenid Empire went far beyond its territorial and military influences and included cultural, social, and religious influences as well. Many other countries adopted Achaemenid customs in their daily lives in a reciprocal cultural exchange. The impact of Cyrus the Great’s Edict of Restoration is mentioned in Judeo-Christian texts and the empire was instrumental in spread of Persian cosmology as far east as China. The Acahemenids fell the invading armies of Alexander the Great.
Alexander the Great defeated the Persian armies at Granicus (334 BC), followed by Issus (333 BC), and lastly at Gaugamela (331 BC). Afterwards, he marched on Susa and Persepolis which surrendered in early 330 BC. Darius III was taken prisoner by Bessus, his Bactrian satrap and kinsman. As Alexander approached, Bessus had his men murder Darius III and then declared himself Darius’ successor, as Artaxerxes V, before retreating into Central Asia. Bessus would then create a coalition of his forces, in order to create an army to defend against Alexander. Before Bessus could fully unite with his confederates at the eastern part of the empire, Alexander, fearing the danger of Bessus gaining control, found him, put him on trial and ordered his execution in a “cruel and barbarous manner”. Alexander generally kept the original Achaemenid administrative structure, leading some scholars to dub him as “the last of the Achaemenids” Upon Alexander’s death in 323 BC, his empire was divided among his generals, the Diadochi, resulting in a number of smaller states. The largest of these, which held sway over the Iranian plateau, was Seleucid Empire, ruled by Alexander’s general Seleucus I Nicator. Native Iranian rule would be restored by the Parthians of northeastern Iran over the course of the 2nd century BC.