The geography of Greece, with its mountainous terrain and many islands, made transportation and unified government difficult. However, it also fostered Greek maritime skills and the establishment of independent city-states. Two major city-states, Athens and Sparta, developed different forms of government - Athens gradually developed a limited democracy while Sparta was a strict military state. These differences came to a head during the Persian Wars in the 5th century BC, when the Greek city-states united to defeat the invading Persian forces in key battles.