After the Greek Dark Ages ancient Greece went from a collection of city-states ruled by aristocracy (from 700 bce) to domination by Sparta and Athens (~510 bce).
The cities had divergent polities. Sparta was a closed, militaristic society ruled by 2 kings. In contrast, Athens moved to a more open model.
To avert becoming a Spartan puppet, Athenian politician Cleisthenes led Athens to limited tribal democracy, where male landholding aristocrats squabbled while Athenian culture flourished, ensuring the city’s eternal fame.
External threats kept Sparta from conquering Athens, at least for a time. Spartan hegemony at the start of 4th century bce was short-lived. Sparta suffered military defeat from other Greek city-states and its helot (slave) population surged while its citizens declined. Spartan citizenship was inherited by blood: men whose numbers were diminished by war. Subjugating the helots and suppressing their revolts weakened the Spartan social fabric. In 479 bce there were an estimated 7 helots for every Spartan.
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The eternal Achilles heel in the ambitions of conquerors is over-extension to the point of collapse. It has been the terminal disease of most empires, abetted by rambunctious upstarts, seeking their turn at enlarging the spoils.
Oppression of the masses and opportunistic territorial conquest by warlords was the well-established mode of civilization by this time. Alexander the Great was Greece’s last huge hurrah in that regard. Then came the Romans.