The mountainous physical geography of Greece helped create several city-states which led to the formation of Athenian democracy, as opposed to a monarchy that ruled over the entire country. Isolated valleys, numerous islands and the Mediterranean Sea influenced the Greeks' choices regarding the settlement of the land.
Geography was not the only factor that drove Greeks to form city-states. A powerful aristocracy kept monarchies from forming, as rich families controlled commerce and government. Greeks shunned small kingdoms in favor of city-state governments from 800 B.C. to 500 B.C.
Most city-states, known singularly as a polis, contained a few hundred to several thousand individuals. Athens was one exception as the thriving commercial center grew to more than 200,000 people, including slaves. Some areas held more than one city-state. The island of Lesbos encompassed five such demographic regions.
The most influential centers of government in ancient Greece included Athens, Sparta and Thebes. Democracy ruled Athens as male citizens aged 20 and older voted on proposals using philosophical methods. Sparta was much more militaristic and went to war with both Athens and Thebes during the Peloponnesian Wars. Thebes dominated Greece after Athens and Sparta declined after a series of wars. When Alexander the Great invaded Persia in 334 B.C., the city-state period of Greece ended, and the influence of Rome began.