In 492 B.C. Darius sent Mardonius, a general who was high in his favour, across the Hellespont with orders to march through Thrace and Macedonia, and having firmly secured their allegiance, to march on to Greece, where Athens and Eretria were to be severely punished for their share in the burning of Sardis. Darius had several reasons for this expedition: the punishment of Athens and Eretria was the one about which most was said, but as it never entered his mind that he could be defeated, Darius probably intended so to destroy the cities on the mainland that the Greeks in Ionia would see that it would be useless to rely on the help of their kinsmen should they ever think of another revolt, and so to punish the European Greeks that they would never dare to interfere again in the affairs of the Persian Empire. But underneath all these reasons were dreams of conquest. The Great King had visions of subduing the whole of Greece and of extending his empire into Europe.
Preparations were made and Mardonius set out. A fleet was to sail close to the coast and to keep in constant touch with the army. All went well until Mount Athos was reached and here a great storm arose.
The north wind handled them very roughly, casting away very many of their ships. It is said that the number of ships destroyed was three hundred, and more than twenty thousand men; for as this sea which is about Athos is very full of sea monsters, some were seized by these and so perished, while others were dashed against the rocks; and some of them did not know how to swim and perished for that cause, others again by reason of cold.
The army fared little better, for it was attacked by some Thracian tribesmen, and though Mardonius forced them to submit to him, he suffered so much loss in the fighting, that as his fleet also had been almost entirely destroyed, he was obliged to depart back to Asia having gained no honour in this undertaking.
Two years went by, and then another expedition was planned. Before it started, Darius sent messengers to Athens and Sparta and other Greek states demanding of them earth and water, the symbols of submission to the Great King. Some states agreed to the demand, either because they were struck dumb with terror at the mere thought that the Great King might invade their land, or because they thought that he was certain to conquer and that by submitting at once they would secure themselves and their land from destruction. Athens and Sparta, however, refused uncompromisingly. The messengers were thrown by the Athenians into a pit, and by the Spartans into a well, and told that if they wanted earth and water they could get them for themselves. On hearing what had happened to his messengers, Darius gave orders to the generals to set out at once for Greece, and to punish every state which had refused submission by enslaving all the inhabitants and bringing them bound to Susa. In particular, Athens and Eretria, for their other misdeeds, were to receive the severest treatment. So the expedition started.
Remembering the disaster at Mount Athos, the fleet sailed this time across the Aegean, touching at Naxos, the first of the Ionian cities that had revolted, and which now received its punishment by being burnt and its people enslaved. Then the Persians went on to Delos, where they offered sacrifices to Apollo, and from there they sailed to Eretria which they immediately attacked. The city held out for six days and then traitors within the walls opened the gates to the Persians who entered and took the city. They burnt it to the ground and carried off all the inhabitants into slavery. And so Eretria was punished for her share in daring to burn a city of the Great King.
News of these movements of the Persians had, of course, reached Athens, but up to this time the Athenians had not considered them as having any dangerous consequences to themselves. But Eretria was not very far from Athens, and when the news of the utter destruction of this city arrived, the Athenians realized the full extent of their peril. Now Hippias, the exiled Tyrant, had taken refuge with the Persians, but he had never given up hope of one day returning to Athens. He was at this very time plotting with friends in the city, and the Persians, knowing this, hoped for their aid in taking Athens. So the Athenians were threatened with dangers both from within and without.
It is easy to imagine the dismay of the Athenians when they heard that Eretria had been destroyed, and that the Persians, the conquerors of the world, were even then on their way to Athens. How could they hope, with their small army, to stand against the great empire? Help must be had, and that quickly. To whom should they turn, if not to Sparta, the foremost military state in Greece, and which, should Athens fall, would be the next state attacked? Self-defence, if no other reason, would surely bring them with speed to Athens. So the Athenians sent Pheidippides, a swift runner, with a message imploring help. The distance from Athens to Sparta is a hundred and fifty miles, and the hours must have seemed very long to the Athenians as they waited for his return with the answer. But so swiftly did he run that he was back before they had dared expect him. How had he done it? For two days and nights he had raced "over the hills, under the dales, down pits and up peaks," and in less than forty-eight hours he had reached Sparta. He rushed into the midst of their Assembly with but scant ceremony, and passionately entreated them to come to the help of Athens.
But to this cry for help the Spartans gave but a cool answer. They would come, certainly, but must wait five days until the moon was full when it was their custom to sacrifice to Apollo, and to break this custom would be to slight the god. When the full moon had come, then they would send help to the Athenians. Pheidippides only waited long enough to receive the answer, and then with despair in his heart, he started back to Athens. Over the hills and the plains, through woods and across streams he raced, appealing in his heart to the gods to whom Athens had ever shown honour, yet who seemed to have deserted her in her utmost need. Was there no help? Suddenly he stopped; whom did he see, sitting in a cleft of a rock? It was Pan, the Goat-God.