In the Battle of Thermopylae, the first battle of the Persian invasion of Greece in the fifth century B.C., the Greek forces were dramatically outnumbered by the Persians. The Greeks were likely to perish.
The Greek cities feared the impending Persian invasion. The Persian forces were well-trained and quite extensive. To give themselves the best chance of survival, the Greeks decided that the Spartans, a group known for their extensive military training, should lead the anti-Persian efforts. The Greeks, with a total of 300 Spartans and 6,000 others, stood to defend the narrow pass of Thermopylae against the nearly 100,000-man Persian army.