In 480 BC, when Xerxes, the son of Darius the Great of Persia, returned to finish the job of conquering the Greeks in which his father had failed, the Athenians consulted the oracle. They were told:
Now your statues are standing and pouring sweat. They shiver with dread. The black blood drips from the highest rooftops. They have seen the necessity of evil. Get out, get out of my sanctum and drown your spirits in woe.
It was unambiguous. When persuaded to seek advice a second time, the oracle gave a way for the Athenians to escape their doom. When Athena approached her father to help her city, Zeus responded that he would grant that “a wall of wood alone shall be uncaptured, a boon to you and your children.”
The oracle again advised the Athenians to flee:
Await not in quiet the coming of the horses, the marching feet, the armed host upon the land. Slip away. Turn your back. You will meet in battle anyway. O holy Salamis, you will be the death of many a woman’s son between the seedtime and the harvest of the grain.
Meanwhile, the Spartans also consulted the oracle and were told:
The strength of bulls or lions cannot stop the foe. No, he will not leave off, I say, until he tears the city or the king limb from limb.
or in a version according to Herodotus:
Hear your fate, O dwellers in Sparta of the wide spaces;
Either your famed, great town must be sacked by Perseus’ sons,
Or, if that be not, the whole land of Lacedaemon
Shall mourn the death of a king of the house of Heracles,
For not the strength of lions or of bulls shall hold him,
Strength against strength; for he has the power of Zeus,
And will not be checked until one of these two he has consumed.
The Spartans withdrew in consternation, wondering which fate was worse. The Delphians themselves then asked how Persia could be defeated. The oracle replied:
Pray to the Winds. They will prove to be mighty allies of Greece.
Events overtook the prophecy when the Persian army assaulted Thermopylae, where the Spartans (notably “the 300”) and allies held the pass against them. The Spartans under King Leonidas (The Lion) resisted the Persian advance at Thermopylae until betrayed by treachery. Refusing to retreat, the entire Spartan contingent, including their King (as foretold), lost their lives, but in so doing gained immortal fame. The Persian armada then sailed to nearby Cape Artemisium, where they were met by the Athenian fleet. The Athenian ships fought against great odds, but in three battles managed to hold their own.
A tremendous storm then arose at Artemesium, with the most violent winds attacking the ships for three days. The Persians lost about 20% of their warships and perhaps the same number of transport vessels to the storm. The stormy winds and huge waves did not harm the Athenian ships.
Back in Athens Themistocles argued that the wall of wood referred to the Athenian navy and persuaded the Athenians to pursue their policy of using wealth from their Attic silver mines at Laurium to continue building their fleet. On the grounds that the oracle referred to the nearby island of Salamis as “holy”, he claimed that those slain would be Greece’s enemies, not the Athenians. For these the oracle would have said “O cruel Salamis”. His voice carried the day, Athens was evacuated to Salamis and in a following naval battle the Athenian fleet and its allies destroyed the Persian fleet at Salamis, while watched by Xerxes. Despite the fact that Athens was burned by the Persians, her occupants were saved, the Persian risk was ended and the authority of the Oracle was never higher.