This shouldn't be a matter of "claims" though. The age of imperialism is supposed to be over.
I'm not talking about Imperialism. I know you're somewhat on the other side of the issue since you're very supportive of the UK being broken up, but as an American I wouldn't just sit by as the US crumbled into independent provinces or suddenly large swaths of territory became independent and governed by Native Americans. I'm cool with the federation that we're rocking, but I wouldn't be in favor of anything less.
Ukraine was 'owned' by a lot of different countries in the past, but prior to WW1 Russia had it for around 100 years. After WW1 it broke into a conglomerate of different states and was then divided between Poland and Russia which became the USSR. The USSR got it back briefly after dividing Poland between themselves and Germany before Germany invaded Russia.
Then WW2 ended and Ukraine's actual status from there on is debatable since it was a member of the USSR. So that's either around 150 years of it being a part of Russia or around 200 years of it being a part of Russia, when before it had never been more than a bunch of different territories that were just ethnically Ukrainian people.
That's what I mean by a claim. There's historical precedence that says "This is the land where Ukrainians live" and historical precedence that says "This was a part of Russian territory for one to two centuries." Only for part of the last century is it actually a place that Ukraine owns.
Then you've got a situation where part of the country doesn't like the other part at all to the point where chances of a violent civil war are still on the table. Given that that part is the part that Russia really cared about in the first place, and the people there apparently very warmly regard Russia, if Russia felt that it was in its interests to annex Crimea from Ukraine I could understand. I wouldn't like it, I wouldn't support it, but I wouldn't think they were just making shit up. Just like I wouldn't think that Ukraine would be making shit up by trying to hold onto a territory that no longer wants to be a part of it (if in fact that is the case and this isn't just a vocal minority or a group 'encouraged' by the Russian government).
Also, I'm not sure that we've talked about how Crimea has tried to become independent or a part of Russia to get away from Ukraine ever since the USSR dissolved in the early 90's.
So yeah, we're out of the Age of Imperialism, but we're not out of the Age of Centralized Power. We haven't gotten to the point where humanity will stop trying to kill itself, and until we are, or until we're closer than we are right now, it is in a country's interest to have a lot of territory and not break up into smaller sovereign states.