Why do you think the Greeks defeated the Persians in the battle of Marathon

It was not the Greeks, it was the Greek city-state of Athens with its allty Plataia.

As their infantry army could not face the Persian cavalry, they waited in the hills until they saw the cavalry being embarked on ships and departing. They ran down to the plain and defeated the infrior Persian infantry.

Realising then where the Persian cavalry was going – to land near Athens and gallop up to the city where traitors would open the gates for them with the Athenian army at Marathon – they ran back over the hills in time to form up in front of the city just as the cavalry disembarked. Frustrated, the Persians reembarked and went home.

The 26 miles the Athenian warriors ran became the basis for today’s Marathon run. The Athenian soldiers did it much tougher, having to carry their weapons and armour as well.