Based on the map why might the Persian Empire have been a threat to Greece?

Persia had already absorbed the Greek city-states in Asia Minor. These states were restless and encouraged revolts against Persia. Persia sent an expedition to bring the main troublemakers, Eretria and Athens under control, but it was defeated at Marathon. Persia then tried to bring all the Greeks under its control to prevent their constant wars, but failed and eventually left them alone to go back to fighting each other.