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Ancient Greece was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity (c. AD 600), that comprised a loose collection of culturally and linguistically related city-states and other territories—unified only once, for 13 years, under Alexander the Great's empire (336-323 BC). In Western history, the era of classical antiquity was immediately followed by the Early Middle Ages and the Byzantine period.