The Greco-Persian Wars had their roots in the conquest of the Greek cities of Asia Minor, and particularly Ionia, by the Achaemenid Persian Empire of Cyrus the Great shortly after 550 BC. The Persians found the Ionians difficult to rule, eventually settling for sponsoring a tyrant in each Ionian city. While Greek states had in the past often been ruled by tyrants, this form of government was on the decline. By 500 BC, Ionia appears to have been ripe for rebellion against these Persian clients. The simmering tension finally broke into open revolt due to the actions of the tyrant of Miletus, Aristagoras. Attempting to save himself after a disastrous Persian-sponsored expedition in 499 BC, Aristagoras chose to declare Miletus a democracy. This triggered similar revolutions across Ionia, extending to Doris and Aeolis, beginning the Ionian Revolt.