While it is commonly believed that only 300 Spartans held off the Persian army at the Battle of Thermopylae, there were actually over 6,400 Greek soldiers initially. After learning of a betrayal that allowed the Persians to flank them, all but 300 Spartans and 700 Thespians retreated to reorganize, leaving those soldiers behind to fight a desperate last stand over 4 days before being overwhelmed and killed.