The first Persian invasion was a response Response to Athenian involvement in the Ionian Revolt , when Athens and Eretria had sent Sending a force to support the cities of Ionia in their attempt to overthrow Removing Persian rule .

In this phase , Sparta , now receiving support from the Achaemenid Empire , supported Supporting rebellions in Athens 's subject states in the Aegean Sea and Ionia , undermining Athens 's empire , and , eventually , depriving the city of naval supremacy .