Ionian Revolt
The first Persian invasion of Greece had its immediate roots in the Ionian Revolt, the earliest phase of the Greco-Persian Wars. However, it was also the result of the longer-term interaction between the Greeks and Persians. In 500 BC the Persian Empire was still relatively young and highly expansionist, but prone to revolts amongst its subject peoples. Moreover, the Persian king Darius was a usurper, and had spent considerable time extinguishing revolts against his rule. Even before the Ionian Revolt, Darius had begun to expand the Empire into Europe, subjugating Thrace, expanding past the Danube river, conquering Paeonia, and forcing Macedon to become a client kingdom to Persia; though the latter retaining an amount of autonomy up to 492 BC. Attempts at further expansion into the politically fractious world of Ancient Greece may have been inevitable. However, the Ionian Revolt had directly threatened the integrity of the Persian empire, and the states of mainland Greece remained a potential menace to its future stability. Darius thus resolved to subjugate and pacify Greece and the Aegean, and to punish those involved in the Ionian Revolt.