The Battle of Thermopylae mópp?lee (Greek: ??????????), detailed primarily by Herodotus, was fought in August 480 BC, between an alliance of Greek city-states and the invading Persian Empire of Xerxes I, at the pass of Thermopylae in central Greece. Vastly outnumbered, the Greeks held up the Persians advance for seven days in total (including three of battle), before the rear-guard was annihilated in one of history's most famous last stands. During two full days of battle, the small force led by King Leonidas I of Sparta blocked the only road through which the massive Persian army could pass. After the second day of battle, a local resident named Ephialtes betrayed the Greeks by revealing a mountain path that led behind the Greek lines. Aware that they were being outflanked, Leonidas dismissed the bulk of the Greek army, remaining to guard the rear with 300 Spartans, 700 Thespian, 400 Thebans and perhaps a few hundred others.

The Persians succeeded in taking the pass but sustained losses disproportionate to those of the Greeks. Nevertheless, in doing so, they conquered Boeotia and Attica, burning Athens in the process. However, the fierce resistance of the Spartan-led army had given the Allies valuable time to prepare the defense of the Peloponnesus, at the isthmus of Cornith, and later that year the Athenian-led navy was able to win a decisive naval battle that would do much to determine the outcome of the war. The Greek victory at the Battle of Salamis prevented a naval invasion of the Peloponnesus, and therefore prevented the completion of the Persian conquest. Demoralised, Xerxes retreated to Asia, leaving a force in Greece under Mardonius to complete the subjugation of the Greeks. The following year, however, a full-strength Allied army defeated the Persian force at the Battle of Plataea, ending the expansion of the Persian Empire into Europe.Bury (1913), page 295.

Both ancient and modern writers have used the Battle of Thermopylae as an example of the superior power of a patriotic army of freemen defending native soil.http://www.victorhanson.com/articles/hanson101106.html The performance of the defenders at the battle of Thermopylae is also used as an example of the advantages of training, equipment, and good use of terrain as force multipliers, and has become a symbol of courage against overwhelming odds.