The greeks were uniting against a common enemy, the Persians. King Darius led the first invasion of Greece in response to Athens interfering in Ionia.
The Battle of Thermopylae 480 BC
We will not let you pass!
The Persians were, surprisingly, defeated by Athens. This was due to better trained soldiers in the Phalanx formation. This lead to a runned named Pheidippides running to spread the new all over Greece, and he died from exhaustion.
Athens Abandoned 479 BC
Darius dies before he could get revenge, but his son Xerxes gathers all of their resources and to invade greece a second time.
The Battle of Salamis 475 BC
No, we lost!
Spartans, led by King Leonidas, held the Persian forces. They were occupied for days allowing other greeks time to prepare.
The Athenians were unable to fight back, so, under the leadership of Themistocles, they abandoned the city and the Persians burned it down.
The Athenians had relocated to a island, and during the Battle of Salamis the Greeks trapped the Persians. The large Persian navy got stuck, allowing the smaller Greek navy to attack and sink the large Persian navy. The Persians were defeated.