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Empire had nearly unlimited resources available, so Darius I appointed another commander named Datis to lead another fleet to Greece. Datis’ specific orders were to “reduce Athens and Eretria to slavery”<ref> Herodotus, Book VI, 94</ref> for their role in the Ionian Revolt. Although Eretria was a much smaller city-state, the neighbor of Athens also sent triremes to support the Ionian Greeks so they also incurred the wrath of Darius I. Instead of following the Aegean coastline as Mardonius so tragically did, Datis decided to take the fleet straight across the Aegean, hopping the many Cyclades islands in the process. Once the Persian fleet arrived in Greece, Datis led it straight to Eretria to teach them a less and send a sign to the Athenians. Herodotus wrote:

Once the Athenians learned of the fate of Eretria, the fear in the city must have been palpable. The Athenians knew that the Persian army was headed in their direction next and that there was little to no deal that could be made. Although Herodotus gives no numbers on the size of the Persian army, all modern scholars agree that it was larger than anything the Athenians could field. A high estimate is around 90,000, although that number includes sailors, <ref> Hammond, N. G. L. “The Campaign and Battle of Marathon.” <i>Journal of Hellenic Studies.</i> 88 (1968) p. 33</ref> while a low estimate puts that number at 12,000 to 15,000 fighting men. <ref> Dpemges, Norman A. “The Campaign and Battle of Marathon.” <i>Historia: Zeitschrift für Alte Geschichte.</i> 47 (1998) p. 6</ref> Whichever number is true, the Athenians were outnumbered and facing an existential threat. But the fear that the Athenians felt after the destruction Eretria may have actually worked to their advantage during the battle because they knew that if they did not win then they would die and their families would be sold into slavery.