Greece is located in Southeast Europe, bordering the Aegean and Ionian seas between the Balkans and Asia Minor. In ancient times, the Greek civilization developed across mountainous terrain in Greece and along the surrounding coasts. The Ancient Greeks lived in independent city-states called poleis, with no unified Greek state. During the Archaic period, Greeks established colonies across the Mediterranean and Black seas due to overpopulation and shortage of land at home. The Classical period saw the rise of Athens and Sparta as the most powerful poleis, and their rivalry led to the Peloponnesian wars in the 5th century BC. Alexander the Great later conquered the Persian Empire and much of Asia Minor and Southwest Asia