Which sea battle of 480 BC saved ancient Greece from Persia?

Salamis, fought in a narrow strait where the Greek fleet had deliberately trapped itself - Themistocles reportedly sent a message to Xerxes telling him the Greeks were about to flee and that he should block both ends, which drew the far larger Persian fleet into water where its numbers could not be used.

The Athenians had built that fleet on an argument three years earlier. A rich silver strike at Laurion produced a windfall that the assembly proposed to distribute to citizens; Themistocles persuaded them instead to spend it on 200 warships, ostensibly against a local rival. Without that vote there is no fleet and no Salamis.

Xerxes watched from a throne on the shore, which is how the Greeks knew the details. The consequences are the sort historians are usually careful about claiming: Persia withdrew, Athens entered the period that produced its drama, philosophy and democracy, and the counterfactual - a Persian-administered Greece - is one of the more genuinely load-bearing what-ifs in European history. Aeschylus fought there, and wrote a play about it from the Persian point of view eight years later, which is a remarkable thing for a veteran to do.