King Darius had found that his hold on the Greek cities of Asia Minor was insecure so long as they could look for armed help to their kindred beyond the Archipelago, and he had sent his satraps to raid the Greek mainland. That first invasion ended disastrously at Marathon. His son, Xerxes, took up the quarrel and devoted years to the preparation not of a raid upon Europe, but of an invasion in which the whole power of his vast empire was to be put forth by sea and land.

It was fortunate for Greece that the man who then counted for most in the politics of Athens was one who recognized the all-importance of sea-power, though it is likely that at the outset all he had in mind was that the possession of an efficient fleet would enable his city to exert its influence on the islands and among the coast cities to the exclusion of the military power of its rival Sparta. When it was proposed that the product of the silver mines of Laurium should be distributed among the Athenian citizens, it was Themistocles who persuaded his fellow-countrymen that a better investment for the public wealth would be found in the building and equipment of a fleet. He used as one of his arguments the probability that the Persian King would, sooner or later, try to avenge the defeat of Marathon. A no less effective argument was the necessity of protecting their growing commerce. Athens looked upon the sea, and that sea at once divided and united the scattered Greek communities who lived on the coasts and islands of the Archipelago. It was the possession of the fleet thus acquired that enabled Themistocles and Athens to play a decisive part in the crisis of the struggle with Asia.

It was in the spring of B.C. 480 that the march from Asia Minor began. The vast multitude gathered from every land in Western Asia, from the shores of the Mediterranean to the Persian Gulf and the wild mountain plateaux of the Indian border, was too numerous to be transported in any fleet that even the Great King could assemble. For seven days and nights it poured across the floating bridge that swayed with the current of the Dardanelles, a bridge that was a wonder of early military engineering, and the making of which would tax the resources of the best army of to-day. Then it marched by the coast-line through what is now Roumelia and Thessaly. It ate up the supplies of the lands through which it passed. If it was to escape famine it must keep in touch with the ships that crossed and recrossed the narrow seas, bringing heavy cargoes of food and forage from the ports of Asia, and escorted by squadrons of long war galleys.

Every Greek city had been warned of the impending danger. Even those who remembered Marathon, the day when a few thousand spearmen had routed an Asiatic horde outnumbering them tenfold, realized that any force that now could be put in the field would be overwhelmed by this human tide of a million fighting men. But there was one soldier-statesman who saw the way to safety, and grasped the central fact of the situation. This was Themistocles the Athenian, the chief man of that city, against which the first fury of the attack would be directed. No doubt it was he who inspired the prophetess of Delphi with her mysterious message that "the Athenians must make for themselves wooden walls," and he supplied the explanation of the enigma.

The Persian must be met not on the land, but in "wooden walls" upon the sea. Victory upon that element would mean the destruction of the huge army on land. The greater its numbers the more helpless would be its position. It could not live upon "the country"; there must be a continual stream of sea-borne supplies arriving from Asia, and this would be interrupted and cease altogether once the Greeks were masters of the sea.

The Athens of the time was not the wonderful city that arose in later years, embellished by the masterpieces of some of the greatest architects and artists the world has ever known. The houses huddled round the foot of the citadel hill—the Acropolis—which was crowned with rudely built primitive temples. But the people whose home it was were startled by the proposal of Themistocles that their city should be abandoned to the enemy without one blow struck in its defence. Not Athens only, but every village and farm in the surrounding country was to be deserted. Men, women, and children, horses and cattle, were all to be conveyed across the narrow strait to the island of Salamis, which was to be the temporary refuge of the citizens of Athens and of the country-folk of Attica.

Would they ever return to their ruined homes and devastated lands, where they would find houses burned, and vines and olives cut down? Could they even hope to maintain themselves in Salamis? Would it not be better to fight in defence of their homes even against desperate odds and meet their fate at once, instead of only deferring the evil day? It was no easy task for the man of the moment to persuade his fellow-countrymen to adopt his own far-sighted plans. Even when most of them had accepted his leadership and were obeying his orders, a handful of desperate men refused to go. They took refuge on the hill of the Acropolis, and acting upon the literal meaning of the oracle toiled with axe and hammer, building up wooden barriers before the gates of the old citadel.

Everywhere else the city and the country round were soon deserted. The people streamed down to the shore and were ferried over to Salamis, where huts of straw and branches rose up in wide extended camps to shelter the crowds that could find no place in the island villages. In every wood on either shore trees were being felled. In every creek shipwrights were busy night and day building new ships or refitting old. To every Greek seaport messages had been sent, begging them to send to the Straits of Salamis as many ships, oarsmen, and fighting men as they could muster.

Slowly the Persian army moved southward through Thessaly. A handful of Spartans, under Leonidas, had been sent forward to delay the Persian advance. They held the Pass of Thermopylæ, between the eastern shoulder of Mount Æta and the sea. It was a hopeless position. To fight there at all with such an insignificant force was a mistake. But the Government of Sparta, slaves to tradition, could not grasp the idea of the plans proposed by the great Athenian. They were half persuaded to recall Leonidas, but hesitated to act until it was too late. The Spartan chief and his few hundred warriors died at their post in self-sacrificing obedience to the letter of their orders. The Persians poured over the Pass and inundated the plains of Attica. The few Athenians who had persisted in defending the Acropolis of Athens made only a brief resistance against overwhelming numbers. They were all put to the sword and their fellow-countrymen in the island of Salamis saw far off the pall of smoke that hung over their city, where temples and houses alike were sacked and set on fire by the victors.

The winds and waves had already been fighting for the Greeks. The Persian war fleet of 1200 great ships had coasted southwards by the shores of Thessaly till they neared the group of islands off the northern point of Eubœa. Their scouts reported a Greek fleet to be lying in the channel between the large island and the mainland. Night was coming on, and the Persians anchored in eight long lines off Cape Sepias. As the sun rose there came one of those sudden gales from the eastward that are still the terror of small craft in the Archipelago. A modern sailor would try to beat out to seaward and get as far as possible from the dangerous shore, but these old-world seamen dreaded the open sea. They tried to ride out the gale, but anchors dragged and hundreds of ships were piled in shattered masses on the shore. Some were stranded in positions where they could be repaired and refloated as the weather cleared up; but by the evening of the third day, when at last the wind fell, only eight hundred galleys of the Persian armada were still in seaworthy fighting condition.