They needed to find new food sources
Answer:
A polis (plural: poleis) was the typical structure of a community in the ancient Greek world. A polis consisted of an urban centre, often fortified and with a sacred centre built on a natural acropolis or harbour, which controlled a surrounding territory (chora) of land. The term polis has, therefore, been translated as ‘city-state’ as there was typically only one city and because an individual polis was independent from other poleis in terms of political, judicial, legal, religious and social institutions and practices, each polis was in effect a state. Like a state, each polis was also involved in international affairs, both with other poleis and non-Greek states in the areas of trade, political alliances and wars. Other cultures had a similar social and political structure, notably, the Babylonians, Etruscans and Phoenicians, and the latter are believed to be the originators of the polis as a communal unit.
The polis emerged from the Dark Ages which followed the fall of the Mycenaean civilization in Greece and by the 8th century BCE a significant process of urbanisation had begun. There were eventually over 1,000 poleis in the Greek World but among the most important were Athens, Sparta, Corinth, Thebes, Syracuse, Aegina, Rhodes, Argos, Eretria, and Elis. The biggest was Sparta, although with some 8,500 km² of territory, this was exceptionally large and most poleis were small in size. However, poleis such as Athens, Rhodes and Syracuse possessed significant naval fleets which also allowed them to control wide areas of territory across the Aegean
