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Why did Darius I plan an invasion of Greece?

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Darius the Great — Darius I This chromolithograph of a frieze at Susa depicts attendants serving Darius I.

Darius I’s decision to invade Greece was driven by a combination of strategic expansionism and a desire for retribution following the Ionian Revolt.

While Darius had long pursued an active expansionist policy—having previously campaigned in Thrace and Macedonia to secure a bridgehead in Europe—the Athenians provided a specific catalyst for his aggression. During the Ionian Revolt (499–494 BCE), in which Greek cities in Asia Minor rebelled against Persian rule, Athens sent limited military assistance to the rebels.123

Following the suppression of the revolt, Darius viewed Athenian involvement as a direct provocation and used it as a pretext for his 490 BCE invasion of Greece. After his forces were defeated at the Battle of Marathon, Darius recognized that the "Greek problem" required a more massive, coordinated military effort, leading him to begin preparations for a larger-scale invasion that was ultimately interrupted by his death in 486 BCE.341