What lessons did Thucydides attempt to convey by the Melian dialog?
I'm not 100%sure, but here goes...
During the Melian Dialogue, Thucydides describes the events of
the discussion that takes place between the Greeks of Athens and
the inhabitants of the island of Melo, Spartan allies. During the
Peloponnesian war, Athens and Sparta created an entire net of
allied cities from which they attained men and war funds (in fact,
the war lasted circa thirty years, from 431 to 404 bC). Forgoing
his usual objectiveness, Thucydides narrates with theatrical pathos
how he believes the dialogue might have taken place, and
demonstrates how the Athenians reacted to diplomatical refusal.
According to Thucydides' reconstruction, before waging war
against the Melians, the Athenians proposed for them to submit
willingly to their control, seeing as they wouldn't be getting any
help from Sparta, their ally, which was too involved in keeping the
homefront protected. Because the Melians refused, the Athenians
then subjected the city to a long and devastating battle, killing
all young men and enslaving all women, children and seniors.
The powerful empire of Athens imposes an impossible chioce to
the inhabitants of Melo, submission or distruction. The Melians try
to subtract themselves from this decision by quoting the classical
arguments of Greek ethics: they invoke universal justice, and they
stress their belief that the Gods will always help those who are
just in the end.
The Athenian response is ruthless. Justice exists only where
there is equality of strenght, otherwise the stronger act in favour
of their greater strength, and the weaker MUST submit to the
There is only ONE eternal law of nature, for men and divinity
alike: "wherever there be strength, there be power".