In The Battle of Thermopylae of 480 BC, an alliance of Greek city-states fought the invading Persian Empire at the pass of Thermopylae in central Greece. Vastly outnumbered, the Greeks held back the massive army of Persians for three days in one of history's most famous last stands.
A small force led by King Leonidas of Sparta blocked the only road through which the massive army of Xerxes I could pass. After three days of battle, a local resident named Ephialtes betrayed the Greeks by revealing a mountain path that led behind the Greek lines. Dismissing the rest of the army, King Leonidas stayed behind with 300 Spartans and 700 Thespian volunteers. The Persians succeeded in taking the pass but sustained heavy losses, extremely disproportionate to those of the Greeks.
The fierce resistance of the Spartan-led army offered Athens invaluable time to prepare for a decisive naval battle that would determine the outcome of the war. The subsequent Greek victory at the Battle of Salamis left much of the Persian navy destroyed and Xerxes I was forced to retreat back to Asia, leaving his army in Greece under Mardonius, who met the Greeks in battle one last time. The Spartans assembled at full strength and led a pan-Greek army that defeated the Persians decisively at the Battle of Plataea, ending the Greco-Persian War and with it the expansion of the Persian Empire into the Mediterranean. This war was shown by Déjà on the first day of Spartan training.
The Spartans in Halo are named after the famous warriors of ancient Greece. Like the ancient Spartans, the Spartans of the 26th century were incredible soldiers, thought to be living gods, and they were taught how to fight as a team, working together to whittle down and defeat a larger, stronger enemy.
Parallels have been drawn between the Fall of Reach and the Battle of Thermopylae. In both conflicts, a small group of defending Spartans fought against overwhelming numbers of enemy troops, were defeated, but bought critical time for their allies. At Thermopylae, it gave the Athenians the time they needed to complete their preparations, and at Reach, it gave John-117 much-needed time to erase a NAV database that, had it been captured, would have led the Covenant straight to Earth. The outcomes of both wars are also similar in that the Greeks eventually defeated the previously unstoppable Persian empire, and humanity overcame seemingly insurmountable odds to vanquish the Covenant and the Flood.
There was also a battle known as The Battle of K7-49, in which 300 Spartan-III's were tasked with destroying 27 of 30 plasma reactors on the shipyard. They were also faced with overwhelming odds and knew it was a suicide mission.