Ancient Greece consisted of several hundred more-or-less independent city states. This was different from other societies, which were tribal, or kingdoms ruling over relatively large territories.
Undoubtedly the geography of Greece—divided and sub-divided by hills, mountains and rivers—contributed to the nature of ancient Greece.
On the one hand, the ancient Greeks had no doubt that they were ‘one people’; they had the same religion, same basic culture, and same language. Yet each city state or “polis” was independent, unification was something rarely discussed by the ancient Greeks.
Later, in the Classical period, the leagues were fewer and larger, and dominated by one city, particularly Athens, Sparta and Thebes. Often cities would be compelled to join under threat of war or as part of a peace treaty.
Some cities were democratic, some were aristocratic, and some were monarchies. Some had many revolutions in which one kind of government replaced another.
(Adapted from Ancient Greece Facts for Kids. Kiddle Encyclopedia. )