But, for the conquest of such a country as Greece, it was requisite, not only that the invader should succeed on land, but also that he should be superior at sea. Xerxes had felt this, and had brought with him a fleet, calculated, as he imagined, to sweep the Greek navy from the Egean. As far as the Pagasaean Gulf, opposite the northern extremity of Euboea, his fleet had advanced without meeting an enemy. It had encountered one terrible storm off the coast of Magnesia, and had lost 400 vessels; but this loss was scarcely felt in so vast an armament. When from Aphetse, at the mouth of the gulf, the small Greek fleet, amounting to no more than 271 vessels, was seen at anchor off Artemisium, the only fear which the Persian commanders entertained was lest it should escape them. They at once detached 200 vessels to sail round the Coast coast of Euboea, and cut off the possibility of retreat. When, however, these vessels were all lost in a storm, and when in three engagements on three successive days, the Greek fleet showed itself fully able to contend against the superior numbers of its antagonist, the Persians themselves could not fail to see that their naval supremacy was more than doubtful. The fleet at Artemisium was not the entire Greek naval force; on another occasion it might be augumented, while their own could scarcely expect to receive reinforcements. The fights at Artemisium foreshadowed a day when the rival fleets would no longer meet and part on equal terms, but Persia would have to acknowledge herself inferior. Meanwhile, however, the balance of advantage rested with the invaders. The key of Northern Greece was won, and Phocis, Locris, Boeotia, Attica, and the Megarid lay open to the Persian army. The Greek fleet could gain nothing by any longer maintaining the position of Artemisium, and fell back towards the south, while its leaders anxiously considered where it should next take up its station. The Persians pressed on both by land and sea. A rapid march through Phocis and Boeotia brought Xerxes to Athens, soon after the Athenians, knowing that resistance would be vain, had evacuated it. The Acropolis, defended by a few fanatics, was taken and burnt. One object of the expedition was thus accomplished. Athens lay in ruins; and the whole of Attica was occupied by the conqueror. The Persian fleet, too, finding the channel of the Euripus clear, sailed down it, and rounding Sunium, came to anchor in the bay of Phalerum.