Darius was now at liberty to take vengeance upon the Athenians. He appointed Mardonius to succeed Artaphernes as satrap in western Asia, and he placed under his command a large armament, with injunctions to bring to Susa those Athenians and Eretrians who had insulted the authority of the Great King. Mardonius, after crossing the Hellespont, commenced his march through Thrace and Macedonia, subduing, as he went along, the tribes which had not yet submitted to the Persian power. He ordered the fleet to double the promontory of Mount Athos, and join the land forces at the head of the gulf of Therma; but one of the hurricanes which frequently blow off this dangerous coast overtook the Persian fleet, destroyed 300 vessels and drowned or dashed upon the rocks 20,000 men. Meantime the land forces of Mardonius had suffered so much from an attack made upon them by a Thracian tribe, that he could not proceed farther. He led his army back across the Hellespont, and returned to the Persian court covered with shame and grief (B.C. 492).

The failure of this expedition did not shake the resolution of Darius. He began to make preparations for another attempt on a still larger scale, and meantime sent heralds to most of the Grecian states to demand from each earth and water as the symbol of submission. Such terror had the Persians inspired by their recent conquest of Ionia, that a large number of the Grecian cities at once complied with the demand; but the Athenians cast the herald into a deep pit, and the Spartans threw him into a well bidding him take earth and water from thence.

In the spring of B.C. 490 a large army and fleet were assembled in Cilicia, and the command was given to Datis, a Median, and Artaphernes, son of the satrap of Sardis of that name. Warned by the recent disaster of Mardonius in doubling the promontory of Mount Athos, they resolved to sail straight across the AEgean to Euboea, subduing on their way the Cyclades. These islands yielded a ready submission; and it was not till Datis and Artaphernes reached Euboea that they encountered any resistance. Eretria defended itself gallantly for six days, and repulsed the Persians with loss; but on the seventh the gates were opened to the besiegers by the treachery of two of its leading citizens. The city was razed to the ground, and the inhabitants were put in chains. From Eretria the Persians crossed over to Attica, and landed on the ever memorable plain of Marathon, a spot which had been pointed out to them by the despot Hippias, who accompanied the army.

As soon as the news of the fall of Eretria reached Athens, a courier had been sent to Sparta to solicit assistance. This was promised; but the superstition of the Spartans prevented them from setting out immediately, since it wanted a few days to the full moon, and it was contrary to their religious customs to commence a march during this interval. Meantime the Athenians had marched to Marathon, and were encamped upon the mountains which surrounded the plain. They were commanded, according to the regular custom, by ten generals, one for each tribe, and by the Polemarch, or third Archon, who down to this time continued to be a colleague of the generals. Among these the most distinguished was Miltiades, who, though but lately a tyrant in the Chersonesus, had shown such energy and ability, that the Athenians had elected him one of their commanders upon the approach of the Persian fleet. Upon learning the answer which the courier brought from Sparta, the ten generals were divided in opinion. Five of them were opposed to an immediate engagement with the overwhelming number of Persians, and urged the importance of waiting for the arrival of the Lacedaemonian succours. Miltiades and the remaining four contended that not a moment should be lost in fighting the Persians, not only in order to avail themselves of the present enthusiasm of the people, but still more to prevent treachery from spreading among their ranks. Callimachus, the Polemarch, yielded to the arguments of Miltiades, and gave his vote for the battle. The ten generals commanded their army in rotation, each for one day; but they now agreed to surrender to Miltiades their days of command, in order to invest the whole power in a single person. While the Athenians were preparing for battle, they received unexpected assistance from the little town or Plataea, in Boeotia. Grateful to the Athenians for the assistance which they had rendered them against the Thebans, the whole force of Plataea, amounting to 1000 heavy-armed men, marched to the assistance of their allies and joined them at Marathon. The Athenian army numbered only 10,000 hoplites, or heavy-armed soldiers: there were no archers or cavalry, and only some slaves as light-armed attendants. Of the number of the Persian army we have no trustworthy account, but the lowest estimate makes it consist of 110,000 men.

The plain of Marathon lies on the eastern coast of Attica, at the distance of twenty-two miles from Athens by the shortest road. It is in the form of a crescent, the horns of which consist of two promontories running into the sea, and forming a semicircular bay. This plain is about six miles in length, and in its widest or central part about two in breadth. On the day of battle the Persian army was drawn up along the plain about a mile from the sea, and their fleet was ranged behind them on the beach. The Athenians occupied the rising ground above the plain, and extended from one side of the plain to the other. This arrangement was necessary in order to protect their flanks by the mountains on each side, and to prevent the cavalry from passing round to attack them in rear. But so large a breadth of ground could not be occupied with a small a number of men without weakening some portion of the line. Miltiades, therefore, drew up the troops in the centre in shallow files, and resolved to rely for success upon the stronger and deeper masses of his wings. The right wing, which was the post of honour in a Grecian army, was commanded by the Polemarch Callimachus; the hoplites were arranged in the order of their tribes, so that the members of the same tribe fought by each other's side; and at the extreme left stood the Plataeans.

Miltiades, anxious to come to close quarters as speedily as possible, ordered his soldiers to advance at a running step over the mile of ground which separated them from the foe. Both the Athenian wings were successful, and drove the enemy before them towards the shore and the marshes. But the Athenian centre was broken by the Persians, and compelled to take to flight. Miltiades thereupon recalled his wings from pursuit, and charged the Persian centre. The latter could not withstand this combined attack. The rout now became general along the whole Persian line; and they fled to their ships, pursued by the Athenians.

The Persians lost 6400 men in this memorable engagement: of the Athenians only 192 fell. The aged tyrant Hippias is said to have perished in the battle, and the brave Polemarch Callimachus was also one of the slain. The Persians embarked and sailed away to Asia. Their departure was hailed at Athens with one unanimous burst of heartfelt joy. Marathon became a magic word at Athens. The Athenian people in succeeding ages always looked back upon this day as the most glorious in their annals, and never tired of hearing its praises sounded by their orators and poets. And they had reason to be proud of it. It was the first time that the Greeks had ever defeated the Persians in the field. It was the exploit of the Athenians alone. It had saved not only Athens but all Greece. If the Persians had conquered at Marathon, Greece must, in all likelihood, have become a Persian province; the destinies of the world would have been changed; and oriental despotism might still have brooded over the fairest countries of Europe.

The one hundred and ninety-two Athenians who had perished in the battle were buried on the field, and over their remains a tumulus or mound was erected, which may still be seen about half a mile from the sea.

Shortly after the battle Miltiades requested of the Athenians a fleet of seventy ships, without telling them the object of his expedition, but only promising to enrich the state. Such unbounded confidence did the Athenians repose in the hero of Marathon, that they at once complied with his demand. This confidence Miltiades abused. In order to gratify a private animosity against one of the leading citizens of Paros, he sailed to this island and laid siege to the town. The citizens repelled all his attacks; and having received a dangerous injury on his thigh, he was compelled to raise the siege and return to Athens. Loud was the indignation against Miltiades on his return. He was accused by Xanthippus, the father of Pericles, of having deceived the people, and was brought to trial. His wound had already begun to show symptoms of gangrene. He was carried into court on a couch, and there lay before the assembled judges, while his friends pleaded on his behalf. They could offer no excuse for his recent conduct, but they reminded the Athenians of the services he had rendered, and, begged them to spare the victor of Marathon. The judges were not insensible to this appeal; and instead of condemning him to death as the accuser had demanded, they commuted the penalty to a fine of fifty talents. Miltiades was unable immediately to raise this sum and died soon afterwards of his wound. The fine was subsequently paid by his son Cimon. The melancholy end of Miltiades must not blind us to his offence. He had grossly abused the public confidence, and deserved his punishment. The Athenians did not forget his services at Marathon, and it was their gratitude towards him which alone saved him from death.

Soon after the battle of Marathon a war broke out between Athens and AEgina. This war is of great importance in Grecian history, since to it the Athenians were indebted for their navy, which enabled them to save Greece at Salamis as they had already done at Marathon. AEgina was one of the chief maritime powers in Greece; and accordingly Themistocles urged the Athenians to build and equip a large and powerful fleet, without which it was impossible for them to humble their rival. There was at this time a large surplus in the public treasury, arising from the produce of the silver-mines at Laurium. It had been recently proposed to distribute this surplus among the Athenian citizens; but Themistocles persuaded them to sacrifice their private advantage to the public good, and to appropriate the money to building a fleet of 200 ships.