Leonidas was a citizen of the ancient Greek city-state of Sparta, and was born about 540 BC. At age seven, he was sent to the agoge, the notoriously harsh and competitive training that the sons of all Spartans had to complete in order to qualify for citizenship and entry into the Spartan army.
As an adult, Leonidas was chosen as commander to lead the combined Greek forces determined to resist the Second Persian Invasion of Greece. In late August, 480 BC, Leonidas went out to meet the vastly larger Persian army numbering approximately 200,000 at a narrow mountain pass named Thermopylae, with a small force of 300 Spartans and about 14,000 other Greeks.
Leonidas and the Greeks successfully repulsed the Persians' frontal attacks for three straight days, dispatching roughly 20,000 of the Persian troops and losing many of their own. The Persian elite unit known to the Greeks as "the Immortals" was held back, and two of the Persian king's brothers died in battle.
On the third day, a traitor among the Greeks named Ephialtes led the Persians by a mountain trail to the rear of the Greeks. At that point Leonidas sent away all Greek troops but remained behind in the pass with his 300 Spartans to delay the Persians, and protect the escape of the other Greeks.
When attacked from both sides, all Spartans, including Leonidas were killed, but his strategy worked; the Persians were delayed enough that all other Greeks escaped. Leonidas and the 300 Spartans are remembered as models of civic duty and self-sacrifice for a purpose greater than themselves.
The Greeks later placed a commemorative stone on top of the burial mound of the Spartans at Thermopylae with the epitaph: "Go tell the Spartans, stranger passing by, that here obedient to their laws we lie."
The story of Leonidas and the 300 Spartans was first told by the ancient Greek Historian Herodotus in The Histories, and is described in the novel Gates of Fire by Steven Pressfield. It is also the subject of the 1962 film The 300 Spartans and the 2012 modern adaptation 300.