There were a dozen city-states involved in the battle, one of which was a contingent from Sparta led by their king Leonidas.
King Leonidas
A coalition of Greek cities led by Sparta.
The invading Persian army and a consortium of southern Greek city-states led by Sparta.
A small force from Greek city-states led by Spartan King Leonidas fought the Persians .
The Persian army versus contingents from southern Greek cities.
The battle of Thermopylae was fought between Greek mainland cities led by Sparta and a Persian invading army. If you have ever seen the movie 300 that's what it is. But there were also about 5,000 non-Spartan troops with the Spartans.
There was a coalition of Greek forces numbering about 4000 at Thermopylae. The coalition was led by King Leonidas and 300 Spartans.
After defeating the doomed Spartan/Greek rear guard led by the Spartan king Leonidas at the narrow pass of Thermopylae the Persians went on to raid the Greek countryside and burned the Greek City-State of Athens . Afterwards , the Persians returned home .
It was prepared for war to defend itself and contribute to and lead the Greek effort by sea and land to oppose the Persian invasion. Thermopylae was a holding operation to force a sea battle in the hope of destroying the Persian navy which threatened the Greek cities. The failure of this plan led to a subsequent sea battle at Salamis which succeeded. Sparta then led the coup de grace in the land battle at Plataia.
Their king, Xerxes I.
Leonidas I was the land commander. Themistocles led the navy
The Battle of Thermopylae was fought between a Greek confederation of soldiers and the armies of the Persian Empire. The Greeks were led by a contingent of elite Spartans aided by thousands of slaves and other Greek soldiers.