While Leonidas was fighting so bravely on land, Themistocles was with the fleet at Artemisium. If the Persians passed this point and entered the Malian Gulf, they would be able to land troops behind Leonidas and secure the pass of Thermopylae without difficulty.
But before the Persian fleet reached Artemisium, a sudden storm arose and dashed some of the ships upon the rocks, some against each other. For three days the tempest raged, and when at length the sea grew calm, four hundred ships had been destroyed.
In spite of this disaster, the Persian fleet was still large enough to alarm the Greeks. When they saw it sailing off the north of the island of Euboea, Eurybiades, the Greek admiral, wished to sail away.
But the inhabitants of the island went to Themistocles to beg him not to let the fleet desert them. So fearful were they, that they offered him thirty talents (about £5800) if he would use his influence to persuade the other admirals to stay and protect their island.
Themistocles readily took the money, and sent eight talents (about £1552) to Eurybiades and his colleagues to bribe them to remain at Euboea.
The next night another storm arose, and again many of the Persian ships were scattered or dashed to pieces on the rocks. But when the wind fell the ships were repaired and the two fleets met in battle.
The struggle was fierce and long, but though the Persians lost a greater number of ships than did the Greeks, yet the fleet under Eurybiades was so heavily damaged that even Themistocles saw that safety lay in retreat. At the same time tidings reached him of the defeat of Thermopylae, and he knew that Xerxes would soon be marching to the south. The fleet must hasten home to protect her own coasts.
So the Greek fleet set sail down the long Euboean strait and did not stop until it reached the island of Salamis. But as they sailed, Themistocles bade the captains of the Athenian fleets send some of their ships to the rocks where the Persians would search for water.
On these rocks Themistocles ordered to be cut in large letters these words, "Ye do wrong, O Ionians, by going against your fathers and bringing Hellas into slavery. If ye can, take our side; if ye cannot, then fight for neither. But if this also is impossible, at least in the battle be slack and lazy, remembering that ye are sprung from us and that we are fighting in a quarrel which ye began."
By these words Themistocles hoped to win the Ionians to his side; or, if that might not be, he hoped at least to make Xerxes so suspicious of them that he would be afraid to let them take part in the battles which had yet to be fought.