Once the Ionian revolt was finally crushed by the Persian victory at the Battle of Lade, Darius began to plan to subjugate Greece.

In 490 BC, he sent a naval task force under Datis and Artaphernes across the Aegean to subjugate the Cyclades, and then to make punitive attacks on Athens and Eretria.

[1] Even before the Ionian Revolt, Darius had begun to expand the Empire into Europe, subjugating Thrace, and forcing Macedon to become a vassal of Persia.

[2] However, the Ionian Revolt had directly threatened the integrity of the Persian empire, and the states of mainland Greece remained a potential menace to its future stability.

[5][6] The Ionian revolt had begun with an unsuccessful expedition against Naxos, a joint venture between the Persian satrap Artaphernes and the Miletus tyrant Aristagoras.

Despite the fact their actions were ultimately fruitless, the Eretrians and in particular the Athenians had earned Darius's lasting enmity, and he vowed to punish both cities.

[14] The revolt was used as an opportunity by Darius to extend the empire's border to the islands of the East Aegean[15] and the Propontis, which had not been part of the Persian dominions before.

[16] The completion of the pacification of Ionia allowed the Persians to begin planning their next moves; to extinguish the threat to the empire from Greece, and to punish Athens and Eretria.

[17] In 492 BC, once the Ionian Revolt had finally been crushed, Darius dispatched an expedition to Greece under the command of his son-in-law, Mardonius.

[18] However, in 490 BC, following up the successes of the previous campaign, Darius decided to send a maritime expedition led by Artaphernes, (son of the satrap to whom Hippias had fled) and Datis, a Median admiral.

[19] After island hopping across the Aegean, including successfully attacking Naxos, the Persian task force arrived off Euboea in mid summer, ready to fulfil their second major objective - to punish Eretria.

[21] However, when these Athenians arrived at Eretria, they were told by a leading citizen, Aeschines, of the divisions amongst the Eretrians, and he advised them to leave and save themselves.

[24] Among ancient sources, the poet Simonides, another near-contemporary, says the campaign force numbered 200,000; while a later writer, the Roman Cornelius Nepos estimates 200,000 infantry and 10,000 cavalry, of which only 100,000 fought in the battle, while the rest were loaded into the fleet that was rounding Cape Sounion;[25] Plutarch[26] and Pausanias[27] both independently give 300,000, as does the Suda dictionary.

[31] Modern historians have proposed wide-ranging numbers for the infantry, from 20,000–100,000 with a consensus of perhaps 25,000;[32][33][34][35] estimates for the cavalry are in the range of 1,000 [32] The Eretrian strategy was to defend their walls, and undergo a siege.

[40] After several days of stalemate, the Athenians finally resolved to attack the Persians, winning a famous victory at the ensuing Battle of Marathon.

[44] In the meantime, Darius began raising a huge new army with which he meant to completely subjugate Greece; however, in 486 BC, his Egyptian subjects revolted, indefinitely postponing any Greek expedition.