The Battle of Thermopylae (480 BCE) was a last ditch effort to stall the Persian army as it marched south toward Athens. Led by Leonidas and his personal guard of 300 Spartans, a citizen army of Greeks was able to delay a Persian army of over 100,000 soldiers at the town of Thermopylae for several days. Although the Greeks were ultimately defeated, the battle provided enough time for the Greek states to regroup and plan a counter attack, eventually defeating the Persians. This battle was crucial not only for the preservation of Greek independence, but also the first democratic society. But how, against all odds, did the small Greek force withstand the Persians for as long as they did? What might have happened had key factors and choices been slightly different? By modeling the battle with historical accuracy, we can simulate various counter-factual scenarios to determine which factors were crucial for the Greeks' incredible achievement.