The Battle of Thermopylae was a large battle fought during the Second Persian War. The two sides were the Persians (led by King Xerxes) and a combination of Greek city-states (led by King Leonidas and the 300 Spartans). It was fought over three days in 480 B.C. in either the month of August or September. After Greece defeated the Persian army in the First Persian War, King Xerxes gathered a massive army/navy and set out to conquer all of Greece. Leonidas found Thermopylae (a narrow coastal pass also known as the "Hot Gates") to be a good place to hold the Persians. The Spartan-led forces were heavily outnumbered. After three days of battle they were defeated. Nearly all of the Greek soldiers who weren't dismissed to go home by Leonidas were killed. The final casualties were 4,000 Greek deaths and 20,000 Persian deaths.