The battle of Thermopylae was fought between 7000 Greeks under Leonidas (including 300 Spartans) and 500,000-2.5 million Persian troops under Xerxes. Xerxes had marched across the Hellespont towards Greece because he wanted Greece to become a satrapy of his and submit to his divine will by giving him earth and water. The two armies met at a 50-foot-wide pass in Northern Greece and here, a small phalanx of Greek hoplites held off wave after wave of Persian infantrymen, cavalrymen, and Immortals. The Spartans demonstrated their bravery again and again, and their lifelong devotion to military training proved to pay off.
Eventually, Ephialtes (a Greek traitor) led the Persians around the pass to the rear of the Greek defenders. Surrounded, the Spartans and Thespians were killed by a volley of arrows. Leonidas, himself, looked forward to dying because the oracle at Delphi prophesized that his death would save Sparta.
For more information:
Histories by Herodotushttp://www.greektexts.com/library/Herodotus/Polymnia/eng/242.html
Extreme War by Terrence Poulos
Military Anecdotes by Max Hastings
Military History Magazine (January 2006): Spartan Stand at ThermopylaeMilitary History Podcast is sponsored by Armchair General Magazine and the International Research and Publishing Corporation