As the Persians mass army conquered country after country there was more land and more soldiers willing to fight for the Persian empire it continued to be like that country after country.
It divided them because the Greek city states were all proud of their own culture, traditions and heritage. This led to many internal conflicts and struggles. At the same time, however, in times of need when there was a strong outside force (Persian empire for example), they joined together as their culture and values bonded them together to fight a common enemy.
