An entity (country / nation / call it what you will) that can be identifed as Ukraine, with a Ukrainian people has been in existance since 882.
The fact that it only most recently achieved its independence in 1991 is frankly neither here nor there. You can say the same for Russia and any other post-Soviet state.
Are you saying they are all fair game for invasion by who pleases to do so?
US and its Allies seem to think that is the case.............
After all US State Department on record about plotting next Ukranian President and who they wished to have.