At the Battle of Thermopylae, is the right answer.
Explanation:
Even though the Greek cities-states fought against each other occasionally when it came to Greek as a whole they united immediately to fight against the foreign invaders. The Persian War is one of the examples of this unity. Athenians crushed the Persians in the battle of Marathon which took place in 490 BC. Subsequently, Xerxes, the then Persian King commenced the second intrusion of Greece, with its ground troops and the navy to completely vanquish Greece. However, the Allied Greek city-states made efforts to prevent the Persian force with the assistant of Spartans soldiers, led by King Leonidas in Thermopylae. In this battle, the Spartan force fought against the Persians to protect the transference of the Greek army.
