The Battle of Thermopylae was fought in 480 BC. The battle was fought between the Persian Empire, led by Xerxes I, and a combination of allied Greek city-states led by Leonidas I of Sparta. There were approximately 7,000 Greek men led by Leonidas I and an estimated 120,000 - 300,000 Persian soldiers led by Xerxes I. The Persian army was able to outflank the Greek army because a local person named Ephialtes informed the Persians that there was an unguarded path that lead behind the Greek lines. The Greek army made one final stand but ultimately were unsuccessful and were over taken by the Persian army.