Persia, under the rule of Darius (r. 522-486 B.C.), was already expanding into mainland Europe and had subjugated Ionia, Thrace, and Macedonia by the beginning of the 5th century B.C. Next in king Darius' sights were Athens and the rest of Greece. Just why Greece was coveted by Persia is unclear. Wealth and resources seem an unlikely motive; other more plausible suggestions include the need to increase the prestige of the king at home or to quell once and for all a collection of potentially troublesome rebel states on the western border of the empire. The Ionian rebellion, the offering of earth and water in submission to the Persian satrap in 508 B.C., and the attack by Athens and Eretria on the city of Sardis in 499 B.C. had not been forgotten either.
Whatever the exact motives, in 491 B.C. Darius once again sent envoys to call for the Greeks' submission to Persian rule. The Greeks sent a no-nonsense reply by executing the envoys, and Athens and Sparta promised to form an alliance for the defense of Greece.

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