The Greek city states usually fought each other numerously, and even helped foreigner invaders against each other, because of the geography of the land. Greece has many mountain ranges, and is separated into a peninsular and islands, which forced many of the Greek-cities to become city-states. This isolation tended to make other cities 'enemies', and made each of them independent. It was not until outside forces (for example, the Persians) that forced them to join together to defeat the opposing foreigners