Simplify the following text for a Grade 8 native English-speaking person.
Original: The first Persian invasion was a response to Greek involvement in the Ionian Revolt , when Athens and Eretria had sent a force to support the cities of Ionia in their attempt to overthrow Persian rule .
Simplified: Then Athens and Eretria had sent a force to support the cities of Ionia in their attempt to overthrow Persian rule .