The Achaemenid Empire (c. 550–330 BCE), sometimes known as First Persian Empire, is an empire in Southwest Asia, founded in the 6th century BCE by Cyrus the Great who overthrew the Median confederation.

It expands to eventually rule over significant portions of the ancient world which at around 500 BCE stretches from the Indus Valley in the east, to Thrace and Macedon on the northeastern border of Greece making it the biggest empire the world had yet seen.

The Achaemenid Empire will eventually control Egypt as well.

It is ruled by a series of monarchs who unify its disparate tribes and nationalities by constructing a complex network of roads.At the height of its power after the conquest of Egypt, the empire encompasses approximately 8 million km2 spanning three continents: Asia, Africa and Europe.

At its greatest extent, the empire includes the modern territories of Iran, Turkey, parts of Central Asia, Pakistan, Thrace and Macedonia, much of the Black Sea coastal regions, Afghanistan, Iraq, northern Saudi Arabia, Jordan, Israel, Lebanon, Syria, and all significant population centers of ancient Egypt as far west as Libya.

It is noted in Western history as the antagonist foe of the Greek city states during the Greco-Persian Wars, for emancipation of slaves including the Jewish people from their Babylonian captivity, and for instituting infrastructures such as a postal system, road systems, and the usage of an official language throughout its territories.

The empire has a centralized, bureaucratic administration under the Emperor and a large professional army and civil services, inspiring similar developments in later empires.

The traditional view is that the Persian Empire's vast size and its extraordinary ethnocultural diversity proves to be its undoing as delegation of power to local governments eventually weakened the king's central authority, causing much energy and resources to be wasted in attempts to subdue local rebellions, explaining why when Alexander the Great (Alexander III of Macedon) invades Persia in 334 BCE he is faced by a disunified realm under a weak monarch, ripe for destruction.This viewpoint, however ,is challenged by some modern scholars, who argue that the Achaemenid Empire was not facing any such crisis around the time of Alexander, and that only internal succession struggles within the Achaemenid family ever came close to weakening the Empire.

Alexander, an avid admirer of Cyrus the Great, eventually caused the collapse of the empire and its disintegration around 330 BCE into what later became the Ptolemaic Kingdom and Seleucid Empire, in addition to other minor territories which gain independence at thistime.

The Iranian Culture of the central plateau, however, continues to thrive and eventually reclaims power by the 2nd century BCE.

The historical mark of the Achaemenid Empire goes far beyond its territorial and military influences and included cultural, social, technological and religious influences as well.

Many Athenians adopt Achaemenid customs in their daily lives in a reciprocal cultural exchange, some being employed by, or allied to the Persian kings.

The impact of Cyrus the Great's Edict of Restoration is mentioned in Judeo-Christian texts and the empire is instrumental in the spread of Zoroastrianism as far east as China.

Even Alexander the Great, the man who set out to conquer this vast empire, respects its customs, by enforcing respect for the royal Persian kings including Cyrus the Great, and even by appearing in proskynesis, a Persian royal custom, despite stern Macedonian disapproval.

The Persian empire also sets the tone for the politics, heritage and history of modern Persia (now called Iran).

The influence also encompasses Persia's previous territories collectively referred to as the Greater Persia.

A notable engineering achievement is the Qanat water management system, the oldest and longest of which is older than 3000 years and longer than 44 miles (71 km.)

In 480 BCE, it is estimated that 50 million people live in the Achaemenid Empire, or about 44% of the world's population at this time, making it the largest ever empire by population in percentage terms.