<P> Having crossed into Europe in April 480 BC, the Persian army began its march to Greece, taking 3 months to travel unopposed from the Hellespont to Therme . It paused at Doriskos where it was joined by the fleet . Xerxes reorganized the troops into tactical units replacing the national formations used earlier for the march . </P> <P> The Allied' congress' met again in the spring of 480 BC and agreed to defend the narrow Vale of Tempe on the borders of Thessaly and block Xerxes's advance . However, once there, they were warned by Alexander I of Macedon that the vale could be bypassed and that the army of Xerxes was overwhelmingly large, thus the Greeks retreated . Shortly afterwards, they received the news that Xerxes had crossed the Hellespont . At this point, a second strategy was suggested by Themistocles to the allies . The route to southern Greece (Boeotia, Attica and the Peloponnesus) would require the army of Xerxes to travel through the narrow pass of Thermopylae . This could easily be blocked by the Greek hoplites, despite the overwhelming numbers of Persians . Furthermore, to prevent the Persians bypassing Thermopylae by sea, the Athenian and allied navies could block the straits of Artemisium . This dual strategy was adopted by the congress . However, the Peloponnesian cities made fall - back plans to defend the Isthmus of Corinth should it come to it, while the women and children of Athens were evacuated to the Peloponnesian city of Troezen . </P> <P> Xerxes's estimated time of arrival at Thermopylae coincided with both the Olympic Games and the festival of Carneia . For the Spartans, warfare during these periods was considered sacrilegious . Despite the uncomfortable timing, the Spartans considered the threat so grave that they dispatched their king Leonidas I with his personal bodyguard (the Hippeis) of 300 men . The customary elite young men in the Hippeis were replaced by veterans who already had children . Leonidas was supported by contingents from the Allied Peloponnesian cities, and other forces that the Allies picked up on the way to Thermopylae . The Allies proceeded to occupy the pass, rebuilt the wall the Phocians had built at the narrowest point of the pass, and waited for Xerxes's arrival . </P> <P> When the Persians arrived at Thermopylae in mid-August, they initially waited for three days for the Allies to disperse . When Xerxes was eventually persuaded that the Allies intended to contest the pass, he sent his troops to attack . However, the Allied position was ideally suited to hoplite warfare, the Persian contingents being forced to attack the Greek phalanx head on . The Allies withstood two full days of Persian attacks, including those by the elite Persian Immortals . However, towards the end of the second day, they were betrayed by a local resident named Ephialtes who revealed to Xerxes a mountain path that led behind the Allied lines . Made aware by scouts that they were being outflanked, Leonidas dismissed most of the Allied army, remaining to guard the rear with perhaps 2,000 men . On the final day of the battle, the remaining Allies sallied forth from the wall to meet the Persians in the wider part of the pass to slaughter as many Persians as they could, but eventually they were all killed or captured . </P>

What were the three battles of the persian war