Battle of Marathon
At Marathon, the Greeks and the Persians fought against each other. The Persians were forced to crowd into a small area to attack the Greeks. This gave the Greeks an advantage. The Greeks managed to attack the Persians 3 or 4 at a time from 3 different angles. They also managed to defend Athens.
The Battle of Marathon was important because if the Greeks did not win, Persia would take over Athens. And leave the Greek city-states vulnerable to conquering.