Hippias?, tyrant of Athens, had been expelled in [510 BC]? by his people, with the assistance of Cleomenes?, King of Sparta?. He fled to the court of Darius to seek assistance.
With the failure of the [Ionic Revolt]? (499 BC - [498 BC]?, Darius was intent on subjugating the Greeks and punishing them for their part in the revolt.
In [492 BC]? Darius dispatched an army under his son-in-law, Mardonius?. This army reduced Thrace? and compelled Alexander of Macedonia to submit again to Persia. However, in attempting to advance into Greece much of the fleet was wrecked in a storm and Mardonius was forced to retreat to Asia.
The strategy discussed in the following is conjecture, by J. Arthur Munro in "The Journal of Hellenic Studies, vol. xix (1899), and G.B. Grundy, in "The Great Persian War" (1901). They do not disagree with Herodotus in regards to the facts of what happened except in the numbers of combatants.
Darius learned through Hippias that the Alcemaeonidae?, a powerful Athenian family, were opposed to Miltiades and ready to help reinstate Hippias. They were also ready to bow to Persian demands in exchange for being excused for their role in the Ionic Revolt. Darius wished to take advantage of this situation to take Athens, which would isolate Sparta and hand him the remainder of the Greeks.
In order for the Athenians to revolt, two things would need to happen. The populace would need to be encouraged to revolt, and the Athenian army would have to leave Athens.
In order to accomplish the first, Darius planned to take Eretria?, which would offer little resistance, and whose fall would terrify the Athenians. To accomplish the second, Darius's army, now led by Artaphernes, son of a satrap of Sardes, and Datis, a Median admiral (Mardonius had been injured in the prior attack), was dispatched in early September, 491 BC to land at the [Bay of Marathon]? and threaten an overland attack towards Athens. This army probably numbered at most 25,000 infantry and 1,000 calvary, since it was transported entirely by sea.
The Persian transports, escorted by the fleet, sailed from Samos to Naxos and reached Carystus on the south coast of Euboea. From there they sailed up the Euboean channel to Eretria, where their aims became clear to the Greeks.
The Eretrians sent an urgent message to Athens for help. The Athenians agreed, but realized they needed more help. They sent a courier to the Spartans and probably a messenger to the Plataeans. The courier arrived in Sparta on September 9th, and the Spartans agreed to help, but pointed out that they could not go to war until the Carneian festival ended on the full moon of September 19-20.
Artaphernes took part of the Persian army and laid siege to Eretria. The remainder of the army crossed with Datis and landed in the [Bay of Marathon]?. The Athenian army, numbering 9,000-10,000, under Callimachus the polemarch and accompanied by his 10 tribal generals marched north from Athens.
When Callimachus heard that the Persians had landed in the Bay of Marathon, he wheeled right and reached the valley of Avlona and encamped his army at the shrine of Heracles. 1,000 Plataeans joined him there.
Since it was obvious from the Persians' disposition that they did not intend to march to Athens, the Athenians waited for the Spartans. For 8 days the armies peacefully confronted each other.
On the ninth day it became known to the Athenians that Eretria had fallen by treachery. This meant that Artaphernes was now free to move, and might attack Athens. On September 21, the Athenian army went out to face the Persians. This was probably a combined decisions by the generals, although Herodotus reports that they were cycling days of command and that Miltiades was in charge at this point, since he had a large part in persuading the others to do so.
Since the bulk of the Persian infantry consisted of archers, the Greek plan was to advance in formation until they reached the limit of the archer's effectiveness, the "beaten zone", or roughly 200 yards, then advance in double time to close ranks quickly and bring their heavy infantry into play. This meant that they would almost certainly end up fighting in disordered ranks, but this was preferable to giving the Persian archers more time. The Greek center was reduced to possibly 4 ranks, from the normal 8, in order to extend the line and prevent the Persian line from overlapping the Greeks. The wings maintained their 8 ranks.
As the Greeks advanced, their wings drew ahead of the center, which was under heavy fire from the archers. As they closed some Persians broke through the resulting gaps and drove the center back in rout. The Greek retreat in the center, besides pulling the Persians in, also brought the Greek wings inwards, shortening the Greek line. The inadvertant result was a double envelopment, and the battle ended when the whole Persian army, crowded into confusion, in panic broke back towards their ships and were pursued by the Greeks.
Some 6,400 Persians died for the loss of approximately 192 Athenians (Herodotus).
As soon as Datis had put to sea, the Athenians marched to Athens. They arrived in time to prevent Artaphernes from securing a landing. Seeing his opportunity lost, Artaphernes set about and returned to Asia.
Marathon was in no sense a decisive victory over the Persians. However, it was the first time the Greeks had bested the Persians on land, and "their victory endowed the Greeks with a faith in their destiny which was to endure for three centuries, during which western culture was born." (J.F.C. Fuller, "A Military History of the Western World").