Next year is the centennial of the independence of Finland, and I would like to learn how to speak of independence of countries in Latin. It seems to me that the Latin words independens and independentia are post-classical. What would be an idiomatic way to say "independent" or "independence" in classical Latin? Although I would prefer an adjective and a noun, any kind of structure will do, as long as it is attested in this meaning.
I would use independens and independentia for this purpose, but I want to find classical alternatives. Or if these two words are indeed classical, I would be happy to be corrected. Words like libertas, emancipare and solutus come to mind, but I'm not sure they have the right spirit. Then there are words like maiestas, potestas, regnum, imperium and res for describing the power of a state, but they alone will not mean independence. Perhaps some combination of these words would work, but I do not know what would be idiomatic.
Could someone provide a passage from classical literature about the independence of a state or other such entity, with a little historical description about the situation?