King Darius had conquered the city of Thrace, so, with Thassos and his continental mines taken, and already preparing to build ships on the coast, he looked at the Athenians and knew that his power over the Greek cities of Asia would only be achieved when conquer European Greece. This is the beginning (or causes) of the conflict between Greeks and Persians, also called the Medical Wars.
Greece and Athens did not seek war with Persia and it wan't possible to reach an agreement with Persia and King Darius, who was already seeking to expand his empire throughout Greece. The battle of Marathon was the great moment for the Greeks, where the Persian power was defeated; then the same Greek generals would fight in other battles and eventually the defeat of Persia would be achieved, preserving the culture, philosophy, art and culture of Classical Greece, which in turn would determine the Western world from the Renaissance to the present day.
