Like locusts the army marched southward through Thrace, eating up the cities as it advanced, for each was required to provide a day's meals for the mighty host. For months those cities had been engaged in providing the food which this army consumed in a day. Many of the cities were brought to the verge of ruin, and all of them were glad to see the army march on. At length Xerxes saw before him Mount Olympus, on the northern boundary of the land of Hellas or Greece. This was the end of his own dominions. He was now about to enter the territory of his foes. With what fortune he did so must be left for later tales. _HOW THE SPARTANS DIED AT THERMOPYLAE._ When Xerxes, as his father had done before him, sent to the Grecian cities to demand earth and water in token of submission, no heralds were sent to Athens or Sparta. These truculent cities had flung the heralds of Darius into deep pits, bidding them to take earth and water from there and carry it to the great king. This act called for revenge, and whatever mercy he might show to the rest of Greece, Athens and Sparta were doomed in his mind to be swept from the face of the earth. How they escaped this dismal fate is what we have next to tell. As one of the great men of Athens, Miltiades, had saved his native land in the former Persian invasion, so a second patriotic citizen, Themistocles, proved her savior in the dread peril which now threatened her. But the work of Themistocles was not done in a single great battle, as at Marathon, but in years of preparation. And a war between Athens and the neighboring island of AEgina had much to do with this escape from ruin. To make war upon an island a land army was of no avail. A fleet was necessary. The Athenians were accustomed to a commercial, though not to a warlike, life upon the sea. Many of them were active, daring, and skilful sailors, and when Themistocles urged that they should build a powerful fleet he found approving listeners. Longer of sight than his fellow-citizens, he warned them of the coming peril from Persia. The conflict with the small island of AEgina was a small matter compared with that threatened by the great kingdom of Persia. But to prepare against one was to prepare against both. And Athens was just then rich. It possessed valuable silver-mines at Laurium, in Attica, from which much wealth came to the state. This money Themistocles urged the citizens to use in building ships, and they were wise enough to take his advice, two hundred ships of war being built. These ships, as it happened, were not used for the purpose originally intended, that of the war with AEgina. But they proved of inestimable service to Athens in the Persian war. [Illustration: THE PLACE OF ASSEMBLY OF THE ATHENIANS.]