What happened
From 499 to 449 BC the Achaemenid Empire and the Greek city-states fought a half-century of intermittent war. Persia crushed the Ionian Revolt (499–493 BC), destroying Miletus in 494 BC, but Darius I’s punitive expedition was defeated by Athens at Marathon in 490 BC. Xerxes I’s massive invasion of 480–479 BC won at Thermopylae — where the Spartan stand under Leonidas became the stuff of legend — and burned Athens, before the decisive Greek naval victory at Salamis (480 BC) and land victory at Plataea (479 BC) turned it back. The Athenian-led Delian League then carried the war into the Aegean, and hostilities wound down by around 449 BC — the so-called Peace of Callias, whose historicity is debated.
Background
The trigger was the Ionian Revolt: the Greek cities of Asia Minor, under Persian rule, rebelled in 499 BC with Athenian support. Persia put the rising down, and Darius I’s punitive expedition against Greece followed.
Consequences
For Persia, the Greek frontier was one theater of a vast empire: the defeats were limited setbacks, and Persia remained the dominant power of the region, later arbitrating Greek affairs with money and diplomacy — as in the King’s Peace of 387 BC. For the Greeks, the wars became the founding epic of Greek identity and the subject of Herodotus’s Histories, the first great work of history. That work is itself a Greek-eyed source, and it is largely through it that the wars are known.