city state vs city-state

city state and city-state both are nouns.

Word	Noun	Adjective	Verb	Adverb
city state	Yes	No	No	No
city-state	Yes	No	No	No

As nouns, city state and city-state are synonyms defined as:

city state (noun)	city-state (noun)
a state consisting of a sovereign city	a state consisting of a sovereign city