A country isn't a country until the United Nations says so?
You nailed it. I would say this: A nation-state is not a nation-state until the United Nations says so. A country is a country when the people there believe they are.
I rather doubt anyone in South Ossetia Declared anything about it becoming "necessary for one people to dissolve the political bands which have connected them with another ..." but the principle's the same.
There was Russia/Soviet Union. Then Georgia broke away from Russia. Then South Ossetia broke away from Georgia and toward Russia. At what point inside this
matreshka doll of independence do we say the impulse is illegitimate?