the two camps built over the winter, so that the ostracism of 482 BC became a direct contest between Themistocles and Aristides. In what has been characterized as the first referendum, Aristides was ostracised, and Themistocles's policies were endorsed. Indeed, becoming aware of the Persian preparations for the coming invasion, the Athenians voted for the construction of more ships than Themistocles had initially asked for. In the run up to the Persian invasion, Themistocles had thus become the foremost politician in Athens. Second Persian invasion of Greece In 481 BC, a congress of Greek city-states was held, during which 30 or so states agreed to ally themselves against the forthcoming invasion. The Spartans and Athenians were foremost in this alliance, being sworn enemies of the Persians. The Spartans claimed the command of land forces, and since the Greek (hereafter referred to as "Allied") fleet would be dominated by Athens, Themistocles tried to claim command of the naval forces. However, the other naval powers, including Corinth and Aegina refused to give command to the Athenians, and Themistocles pragmatically backed down. Instead, as a compromise, the Spartans (an insignificant naval power), in the person of Eurybiades were to command the naval forces. It is clear from Herodotus, however, that Themistocles would be the real leader of the fleet. The 'congress' met again in the spring of 480 BC. A Thessalian delegation suggested that the allies could muster in the narrow Vale of Tempe, on the borders of Thessaly, and thereby block Xerxes's advance. A force of 10,000 hoplites was dispatched under the command of the Spartan polemarch Euenetus and Themistocles to the Vale of Tempe, which they believed the Persian army would have to pass through. However, once there, Alexander I of Macedon warned them that the vale could be bypassed by several other passes, and that the army of Xerxes was overwhelmingly large, and the Greeks retreated. Shortly afterwards, they received the news that Xerxes had crossed the Hellespont. Themistocles now developed a second strategy. The route to southern Greece (Boeotia, Attica and the Peloponnesus) 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. However, after the Tempe debacle, it