Branko Milanovic
Without going into the arguments of self-determination etc. that this article does: https://thinkbrics.substack.com/p/crimea-why-this-peninsula-remains one needs to focus on what legally unites Crimea and Kosovo (and what Zelensky clearly recognizes) is the following. The break-up of USSR, Czechoslovakia and Yugoslavia happened along the republican borders which then suddenly became international borders. Both Kosovo and Crimea were "autonomous province" and "autonomous republic" within respectively Serbia and Ukraine. It is important to know that in Communist federalism the term "autonomous" was used for political entities whose national composition was different from the national composition of the Union Republic within which they were located. There were other Soviet republics that also had "autonomous republics" (e.g. Russia had eleven of them, Georgia, three; Azerbaijan 1). In Yugoslavia, only Serbia had 2. CSSR had no "autonomous region" or "republic". (Note that the term "autonomous" is used in China for exactly the same purpose --to indicate a political unit with a national minority-- as in USSR & SRFJ. But China is not legally defined as a federal country--while the other three were.) The problem is that once you move the right to independence to the level below the republican level, you legally open the Pandora's box where it should end. Should all "autonomous regions" have the right to statehood based on (say) referendums? From the Western point of view only Kosovo has received that privilege; Abkhazia & South Osetia (whose legal position is identical) are not recognized by the West.