The Persian army advanced without opposition as far as the pass of Thermopylae, which guarded the only road into the rest of Greece. Twelve days after the army had started on its march the great fleet crossed the AEgean to establish contact with the army and bring supplies. The army was checked by the valor of Leonidas, and the Persian fleet was intercepted by a Greek fleet which stood guard over the channel leading to the Gulf of Lamia, thus protecting the sea flank of Leonidas. The Persian fleet, after crossing the open sea safely, made its base at Sepias preparatory to the attack on the Greek fleet. The latter numbered only about 380 vessels to some 1200 of their enemy and the prospects for the Persian cause looked bright indeed. But as the very number of the Persian ships made it impossible to beach all of them for the night a large proportion of them were anchored, lying in eight lines, prows toward the sea. At dawn a northeast gale fell upon them, and, according to the Greek accounts, wrecked 400 triremes, together with an uncounted number of transports. Meanwhile the Greek ships had taken refuge under the lee of the island of Euboea, and the news of the Persian disaster was signaled to them by the watchers on the heights. [Illustration: SCENE OF PRELIMINARY NAVAL OPERATIONS, CAMPAIGN OF SALAMIS] As soon as the weather moderated the Greeks returned to their position in the straits near Artemisium, and during the next three days the two fleets fought stubbornly but without advantage to either side. During the second day a southerly gale caught a flying squadron of some 200 triremes, that had been dispatched round the island of Euboea to catch the Greeks in the rear, and not one of the Persian ships survived. The Greek rear guard squadron of fifty brought the welcome news to the main fleet and served as a much needed reenforcement. Although the Persian armada had lost about half its force in three days by storms, the odds were still so heavily against the Greeks that they found themselves in constant peril of having their flanks turned in this open sea fighting. On the afternoon of the third day the pass of Thermopyae was forced, thanks to the treachery of a Greek and the contemptible policy of the Spartan government which steadily refused the plea of Leonidas for reenforcements. With Thermopyae taken there was no further reason for the Greek fleet to try to hold the straits north of Euboea, and during the night it retired unobserved. The following day the Persian fleet advanced and brought to the army the supplies which it sorely needed.