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September 480 BC: Battle of Salamis

Victory at Thermopylae meant that all Boeotia fell to Xerxes; Attica was then open to invasion. The remaining population of Athens was evacuated, with the aid of the Allied fleet, to Salamis. The Peloponnesian Allies began to prepare a defensive line across the Isthmus of Corinth, building a wall, and demolishing the road from Megara, abandoning Athens to the Persians. Athens thus fell to the Persians; the small number of Athenians who had barricaded themselves on the Acropolis were eventually defeated, and Xerxes then ordered the destruction of Athens.

The Persians had now captured most of Greece, but Xerxes had perhaps not expected such defiance; his priority was now to complete the war as quickly as possible. If Xerxes could destroy the Allied navy, he would be in a strong position to force an Allied surrender; conversely by avoiding destruction, or as Themistocles hoped, by destroying the Persian fleet, the Allies could prevent conquest from being completed. The Allied fleet thus remained off the coast of Salamis into September, despite the imminent arrival of the Persians. Even after Athens fell, the Allied fleet remained off the coast of Salamis, trying to lure the Persian fleet to battle. Partly because of deception by Themistocles, the navies met in the cramped Straits of Salamis. There, the Persian numbers became a hindrance, as ships struggled to maneuver and became disorganised. Seizing the opportunity, the Allied fleet attacked, and scored a decisive victory, sinking or capturing at least 200 Persian ships, therefore ensuring the safety of the Peloponnessus.

According to Herodotus, after the loss of the battle Xerxes attempted to build a causeway across the channel to attack the Athenian evacuees on Salamis, but this project was soon abandoned. With the Persians' naval superiority removed, Xerxes feared that the Allies might sail to the Hellespont and destroy the pontoon bridges. His general Mardonius volunteered to remain in Greece and complete the conquest with a hand-picked group of troops, while Xerxes retreated to Asia with the bulk of the army. Mardonius over-wintered in Boeotia and Thessaly; the Athenians were thus able to return to their burnt-out city for the winter.