Go Tell The Spartans

Go tell the Spartans, stranger passing by
That here, obedient to their laws, we lie.

On or about this date in 480 BC the Battle of Thermopylae took place.  In a narrow pass King Leonidas of Sparta led a force of almost 7,000 Greeks against a huge Persian army led by the Emperor Xerxes.  The Persians, rulers of the Middle East, had crossed the Bosporus and were moving south with the intention of obtaining the submission of the Greek states and destroying Athens.  At the end of the second day after learning that his force had been outflanked, Leonidas dismissed most of the Greek allies and remained with a rear guard made up of the entire Spartan contingent of 300 plus 400 Thebans and 700 Thespians.  All of the Spartans and most of the other Greeks died.

Simonides of Ceos (556BC-468BC) composed the famous epigram which was engraved on a stone atop the Spartan burial mound at the site of the battle.  The second line is a reference to the requirement of Spartan law that its soldiers return victorious or die in battle.  The epigram has been translated in many different ways; the one above is the one I like best.
(from japanfocus.org)
Though the Persians advance was slowed by Thermopylae as well as by the naval Battle of Artemisium which took place nearby at the same time and which inflicted substantial losses on the Persian fleet, they continued their advance and occupied and burned Athens.  At this desperate moment Athenian navy lured the Persians into a trap and demolished their fleet at the Battle of Salamis.  Though the Persian army did not retreat Salamis ended the threat of annihilation for the Greeks. The following year a large Spartan led coalition routed an even bigger Persian army at the Battle of Plataea liberating the occupied portions of Greece.

Thermopylae is one of the founding legends of Western civilization.  The sacrifice of the 300 (unfortunately the role of the allies has been downplayed) has resounded through the ages.  It was celebrated during the Classical Era (before 500AD) and in the modern era movies such as Go Tell The Spartans (about the Vietnam War) have referenced the event while films such as the recent 300 tell the story in a different and only remotely accurate way to a new audience.