|There is more information available on this subject at Battle of Thermopylae on the English Wikipedia.|
The Battle of Thermopylae was fought between an alliance of Greek city-states, led by Sparta, and the Persian Empire of Xerxes I over the course of three days, during the second Persian invasion of Greece. The Spartans who led the defense were all killed, and they may have known in advance that they would be, but their courage provided inspiration to the Greeks. Although the Spartans were killed at Thermopylae, they held the line long enough for others to escape and the following year the Greeks did win battles they fought against the Persians.
Relevance in the Halo universe
The SPARTAN-IIs were taught about the battle on their first day of training by the AI Déjà. Master Chief Petty Officer John-117 also references the battle many times during his service, such as when he compares the Massacre at Eridanus Secundus to the Battle of Thermopylae in response to Vice Admiral Danforth Whitcomb's comparison of it to the Battle of the Alamo.
The UNSC Thermopylae was also named after the battle.
Parallels have been drawn between the Battle of Thermopylae and the Fall of Reach. In both conflicts, a small group of defending spartans who fought against an overwhelming enemy force, were in the end defeated, but not without buying their respective allies; The Athenians and John-117, critical time to complete their preparations for the wars. At Reach, it gave John-117 the required time to erase a NAV database that, had it been captured, would have lead the Covenant Straight to Earth.
Between July 27 and August 2, 2537 There was also the mission known as Operation: PROMETHEUS, in which 300 Spartan-IIIs were tasked with destroying 27 of 30 plasma reactors held in a Covenant shipyard on the volcanic asteroid known as K7-49 . They were also faced with overwhelming odds and knew it was a suicide mission.