Final answer:
Darius I initiated the first Persian invasion of mainland Greece, fueled by Athenian support of the Ionian Rebellion against Persian rule.
Step-by-step explanation:
The first Persian invasion of mainland Greece was launched by the Persian ruler Darius I. The earlier conquests by Cyrus the Great laid the groundwork for direct conflict when Persia subdued the Greek city-states of Ionia around 546 BCE. Dissatisfaction with Persian-appointed tyrants led to the Ionian Rebellion in 499 BCE, which was supported by Athens among others. The Persian ruler at the time, Darius decided that Athens must be punished for its intervention, planning a full-scale invasion of Greece. This spurred the series of conflicts known as the Persian Wars, starting from 492 BCE to 449 BCE.