other contemporary state. By early 480 BC, the preparations were complete, and the army which Xerxes had mustered at Sardis marched towards Europe, crossing the Hellespont on two pontoon bridges. The Athenians had also been preparing for war with the Persians since the mid-480s BC, and in 482 BC the decision was taken, under the guidance of the Athenian politician Themistocles, to build a massive fleet of triremes that would be necessary for the Greeks to fight the Persians. However, the Athenians did not have the manpower to fight on land and sea; and therefore combatting the Persians would require an alliance of Greek city states. In 481 BC, Xerxes sent ambassadors around Greece asking for earth and water, but made the very deliberate omission of Athens and Sparta. Support thus began to coalesce around these two leading states. A congress of city states met at Corinth in late autumn of 481 BC, and a confederate alliance of Greek city-states was formed. It had the power to send envoys asking for assistance and to dispatch troops from the member states to defensive points after joint consultation. This was remarkable for the disjointed Greek world, especially since many of the city-states in attendance were still technically at war with each other. Initially the 'congress' agreed to defend the narrow Vale of Tempe, on the borders of Thessaly, and thereby block Xerxes's advance. However, once there, they were warned by Alexander I of Macedon that the vale could be bypassed through the pass by the modern village of Sarantaporo, and that the army of Xerxes was overwhelming, so the Greeks retreated. Shortly afterwards, they received the news that Xerxes had crossed the Hellespont. A second strategy was therefore adopted by the allies. The route to southern Greece (Boeotia, Attica and the Peloponnese) would require the army of Xerxes to travel through the very 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, whilst the women and children of Athens had been evacuated en masse to the Peloponnesian city of Troezen. Famously, the much smaller Greek army