city-states are very important to many people
A greek city-state was called a polis, which is where the word Metropolis comes from.
Whenever a sentence requires commas that would make the series commas seem redundant or confusing the correct punctuation is the semi-colon, e.g., Portland, Birmingham, and Washington can be written...
Comma after the city. ex: I was born in Baltimore, Maryland. My family and I are taking a vaction trip to Miami, Florida.
because they were independent