The 300-Plus at Thermopylae
To be held in RESERVE and used as needed
The Greco-Persian Wars had started in 499 BCE, when some Greek city-states supported colonists rebelling against the Persian Empire in Ionia (modern Turkey). In that First Invasion of Greece, Persian Emperor Darius I was defeated at the famed Battle of Marathon in 490.
But after Darius's death, his son Xerxes I returned for a second, retributive, invasion. As Xerxes's land force advanced toward Thessaly, his fleet paralleled its course along the north coast of the Aegean. The Persian navy met the Greeks in the Straits of Artemesium; simultaneously, the massed Greek armies held the narrow pass at Thermopylae, a similarly defensible choke-point said by the historian Herodotus to be only "wide enough for a single carriage" to pass.
About 7,000 men under King Leonidas of Sparta held the pass against what ancient sources claim was a million Persians, but modern scholars consider to have been between 100,000 and 150,000. Still, vastly outnumbered, the Greeks held out for seven days, three of which were consumed in open battle.
The outcome of the battle turned on the treachery of a single man. Hoping for a reward from the Persians, a local man named Ephialtes--the name has become a byword for "traitor" in Greek, as Judas is used in English--showed the Persians a path that circumvented the pass.
Seeing the situation, Leonidas dismissed most of the Greeks, keeping 300 Spartans with him, as well as 700 Thespians, up to 900 enslaved Spartan helots, and 400 Thebans. The Greeks were killed to a man. (In some popular media, it is the "300" alone who battle to the death.)
When the leader of the Greek navy heard of the defeat at Thermopylae, he withdrew from Artemisium. Athens was subsequently taken by the Persians, but the Greek navy dealt the Persians a resounding defeat at Salamis later in the year. (Thus, Ephialtes never received his reward.) With his escape route compromised, Xerxes retreated to Asia; the remains of his army in Europe under his general, Mardonius, were finally defeated at the Battle of Plataea in August of 479, bringing an end to the presence of the Persians in Europe. The Greco-Persian Wars would end three decades later.