The Greek city-states known as poleis developed various forms of government from monarchy to democracy. Tyrants could gain power by seizing control with mercenaries but also support reforms benefiting citizens. In Athens, Solon's reforms reduced slavery and opened political offices to more citizens. However, aristocratic infighting continued until Cleisthenes fully established democracy, granting all male citizens equal political rights. Meanwhile, the Persian Empire posed a threat that would unite the Greeks.