The Achaemenid Empire, founded by Cyrus the Great in 550 BCE, was based in what is now Iran and became the largest empire of its time, covering 5.5 million square kilometers. It extended from the Balkans and Egypt in the west, across West Asia, Central Asia, and into the Indus Valley in South Asia.^[17]

Originating in Persis, southwestern Iran, around the 7th century BCE, the Persians,^[18] under Cyrus, overthrew the Median, Lydian, and Neo-Babylonian Empires. Cyrus was noted for his benign governance, which contributed to the empire's longevity, and was titled "King of Kings" (shāhanshāh). His son, Cambyses II, conquered Egypt, but died amidst mysterious circumstances, leading to Darius I’s rise to power after overthrowing Bardiya.

Darius I established administrative reforms, built extensive infrastructure like roads and canals, and standardized coinage. The Old Persian language was used in royal inscriptions. Under Cyrus and Darius, the empire became the largest in history up to that point, known for its tolerance and respect for other cultures.^[19]

In the late sixth century BCE, Darius extended the empire into Europe, subduing regions including Thrace and making Macedon a vassal state around 512/511 BCE.^[20] However, the empire faced challenges in Greece. The Greco-Persian Wars began in the early 5th century BCE following a revolt in Miletus supported by Athens. Despite early successes, including the capture of Athens, the Persians were eventually defeated and withdrew from Europe.^[21]

The empire's decline began with internal strife and external pressures. Egypt gained independence in 404 BCE after Darius II's death but was reconquered in 343 BCE by Artaxerxes III. The Achaemenid Empire ultimately fell to Alexander the Great in 330 BCE, marking the start of the Hellenistic period and the rise of the Ptolemaic Kingdom and Seleucid Empire as successors.

In the modern era, the Achaemenid Empire is acknowledged for establishing a successful model of centralized, bureaucratic administration. This system was characterized by its multicultural policy, which included the construction of complex infrastructures like road systems and an organized postal service. The empire also promoted the use of official languages across its vast territories and developed extensive civil services, including a large, professional army. These advancements were influential, inspiring similar governance styles in various empires that followed.^[22]