Even if the Ukraine government is found illegitimate, russia has no right to invade a sovereign country. Putin is acting as if he is a party to this, but he no legal standing himself.
He claims he is not a party and technically Russia has not used armed force in Ukraine.
None of what happened in the last two weeks fits normal rules of warfare. We’re punishing the Russians because we’re too proud to admit they outsmarted us.
Just like Iraq?
If the radical government in Kiev is found to be illegitimate (and it is), than the old elected government is still the legitimate authority. Russian actions can be presented as being taken at their request.
You'd like to believe that things like Constitutional law and democratic process still matter to the West. But after listening to Victoria Nuland on the telephone casually discussing which unelected opposition leader the US wants to put in charge in Kiev, over the objections of the European Union's choice, I'm not so sure.