briefly, then:
1) unilateral declarations of independence may be unconstitutional in individual countries. however,
2) international law is that the right to declare independence trumps everything and
3) liberal courts have generally agreed that democratic states should not use force to repress established independence movements.
4) the referendum in crimea is consequently illegal under ukrainian law but legal under international law. further, it would be contrary to international norms for ukraine to react with force to suppress an independence movement.
however, the reality is that neither side cares about national or international law.
as far as i can tell, the western media line that the referendum was illegal and should not be recognized is rejecting international law on the subject, which is that sovereign states have no rights whatsoever in preventing independent movements.
people are talking kosovo, but the example i understand best is quebec. (yes, i've studied this formally).
there's been a subtle supreme court ruling on the matter in canada, which is best to toss in the garbage as utter bullshit. those are strong words and i mean them. it's that bad. really. the succession reference case in canada is actually a breathtaking example of blatant historical whitewashing and revisionism to create a trumped up pretext to uphold state interests. it is now all but illegal for quebec to separate from canada, but if they decide to do it anyways is it legal to stop them?
this is where the ruling becomes subtle. it's all but illegal to separate, yet equally illegal to try and stop them, should they succeed in declaring independence. further, a successful declaration of independence is defined by the ability to assert sovereignty, which is essentially the ability to prevent canada from stopping them. the ruling consequently purposefully states nothing of value about quebec's functional right to unilateral succession (despite being clear it would be all but illegal in canadian law), but it does put restrictions on canada's ability to prevent it by force.
on that point, the court did pick out the more interesting question. anybody can claim anything. who that opposes this is going to stop them? the more pressing question is what kind of rights exist in suppressing independence movements, and when worded like that it becomes clear that these rights must be limited or perhaps even non-existent. regardless of what crimea's rights may or may not be, to what extent is ukraine justified in actively preventing an expression of sovereignty? the reference case's rejection of a right of canada to use force to reassert sovereignty essentially negates the relevance of the canadian constitution, should quebec decide to forcefully declare it's own interests as an expression of it's own will. i couldn't see how a ukrainian court of any remotely liberal lean could rule differently over crimea.
the international law is consistent with this position. in international law, declarations of independence trump everything else. they certainly trump constitutions. i think the above discussion provides some context as to why that is.
now, that's not to deny that something sneaky has become apparent over time. the russians have not been transparent about this. in order to regain lost credibility with me, they would have to reject the referendum and reassert crimea's place in ukraine. this was their initial position, which they've now flopped on. the position is extremely important in the context of them moving military people to the region - it's one thing to temporarily invade to protect a base, it's another to invade with the goal of annexation. rejecting the request to join russia is not the same thing as rejecting the validity of the referendum. and pointing out russian dishonesty is not the same thing as unilaterally rewriting international law. it's consistent to both disagree with the russian position on the annexation and uphold the legality of the vote. that seems to be the correct position.
briefly, then:
1) unilateral declarations of independence may be unconstitutional in individual countries. however,
2) international law is that the right to declare independence trumps everything and
3) liberal courts have generally agreed that democratic states should not use force to repress established independence movements.
4) the referendum in crimea is consequently illegal under ukrainian law but legal under international law. further, it would be contrary to international norms for ukraine to react with force to suppress an independence movement.
however, the reality is that neither side cares about national or international law.