Greek city-states were a collection of cities that emerged around 500 BCE. The cities of Greece were unique in the ancient world because they did not develop into larger empires Egypt or Rome. Separated by the mountainous geography of mainland Greece, each city-state developed separate cultures with a few similarities. Around 700 BCE, Hellenism, encompassing Greek politics, religion, and culture, remained the essential connection of the city-states. These individual cities fought one another constantly, except when outside invaders like the Persians threatened their way of life in the early 400s BCE. Later, internal Greecian conflicts weakened the mainland, and it fell under the control of the Macedonians by the mid-300s BCE and the Romans only a century later. Thereafter, the Greeks were physically conquered by Rome but continued to impact the world through arts and politics.