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geographic region, south-central Iran. The ancient region, known as Pars, or Persis (q.v.), was the heart of the Achaemenian empire (559-330 BC), which was founded by Cyrus the Great and had its capital at Pasargadae. Darius I the Great moved the capital to nearby Persepolis in the late 6th or early 5th century BC. Alexander the Great defeated the Achaemenian army at Arbela in 331 and burned Persepolis. Persis (Fars) became part of the Seleucid kingdom in 312 after Alexander's death. The Parthian empire (247 BC-AD 224) of the Arsacids (corresponding roughly to the modern province of Khorasan in Iran) replaced the rule of the Seleucids in Persis during 170-138 BC. The Sasanid empire (AD 224-651) had its capital at Istkhr. Not until the 18th century, under the Zand dynasty (1750-79) of southern Iran, did Fars again became the heart of an empire, this time with its capital at Shiraz. In the 20th century the role of Fars in Iran declined considerably with the building of the Trans-Iranian railway outside the region and the discovery of oil in Khuzestan province