Why did the Persians and the Greeks react?

Answer 1

Prtsia had incorporated the Greek city-states in Asia Minor into its empire. These cities had been established by mainland Greek cities, and when they revolted, sought help from their mother cities. Persia realised this as a source of ongoing trouble and decided that the only way to bring peace was to bring all the Greek city-states under control of Persian provincial governors and appoit local Greek tyrants to rule the cities. Some of the cities agreed, others resisted, causing the Persians to invade to enforce their policy.