The Achaemenid Empire or Achaemenian Empire (/əˈkiːmənɪd/; Old Persian: 𐎧𐏁𐏂, Xšāça, lit. 'The Empire' or 'The Kingdom'), was the ancient Iranian empire founded by Cyrus the Great in 550 BC; the First Persian Empire. Based in Western Asia, it was the largest empire the world had ever seen at its time, spanning a total of 5.5 million square kilometres (2.1 million square miles) from the Balkans and Egypt in the west to Central Asia and the Indus Valley in the east.
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Around the 7th century BC, the region of Persis in the southwestern portion of the Iranian plateau was settled by the Persians. From Persis, Cyrus rose and defeated the Median Empire as well as Lydia and the Neo-Babylonian Empire, marking the formal establishment of a new imperial polity under the Achaemenid dynasty.
In the modern era, the Achaemenid Empire has been recognized for its imposition of a successful model of centralized, bureaucratic administration; its multicultural policy; building complex infrastructure, such as road systems and an organized postal system; the use of official languages across its territories; and the development of civil services, including its possession of a large, professional army. Its advancements inspired the implementation of similar styles of governance by a variety of later empires.
By 330 BC, the Achaemenid Empire was conquered by Alexander the Great, an ardent admirer of Cyrus the Great; the conquest marked a key achievement in the then-ongoing campaign of his Macedonian Empire. As Alexander's death triggered the beginning of the Hellenistic period, most of the territory of the fallen Achaemenid Empire came under the rule of the Ptolemaic Kingdom and the Seleucid Empire, both of which had emerged as successors to the Macedonian Empire following the Partition of Triparadisus in 321 BC. Hellenistic rule remained in place for almost a century before the Iranian elites of the central plateau reclaimed power under the Parthian Empire.