Nice post, however in this bit the analogy doesn't really seem to fit as Germany can't use that argument about East Prussia today, nor could Finland use that argument about the areas lost in 1944 because Germany and Finland were on the losing side in OTL and there hasn't been any major event (i.e. another major conflict or at least substantial unopposed military action) to reverse this and their acceptance of such. In TTL, Russia was on the losing side...for a while. Hence Karelia was lost to Finland. In a European order that was maintained by a victorious Nazi Germany (thus whatever arguments Finland did use to maintain a claim on Karelia, there would always be the problem that they finally fully obtained Karelia only during a period of Nazi domination and basically in alliance with (or more diplomatically co-belligerence with) Nazi Germany). But then Nazi Germany collapsed and another war ensued (i.e. Round 2 of sorts) in which Russia and the West Slavs liberated themselves from Nazi control (i.e. reversing the order maintained by the Nazis). And unlike in OTL where the Soviet Union collapsed a full 47 years after Germany lost East Prussia and Finland lost some territories (which means anyone who as even 20 years old at the time in those areas would have been nearly 70 in 1991), here Nazi Germany collapses in 1960. That is just 14-15 years after the Soviet Union lost Karelia. It wouldn't be several decades. It wouldn't even be two decades fully. Even stretching it out to when Russia is fully reunited under the Soviet government in 1972 ITTL that's just 27 years. Most people who were expelled from Karelia in 1945 are quite likely alive and very much remember their homes. It also seems quite a stretch that between 1960 and 1972, there wouldn't have been a partially successful attempt to regain Karelia in light of the fact that none of the western Allies are likely to actually oppose Russian attempts to regain it since they would be very unlikely to want to legitimize Finnish control over the area because they may associate it with the Nazi era of domination of Europe (even if they don't actually oppose Finnish control and even if that dispute pre-dated the Nazis in origin and was mostly independent of it). I would expect at the very least that between 1960 and 1972 in TTL that at least parts of Karelia would have been reoccupied by Russia even if they don't fully re-take it. It also seems far more likely that even with the Finnish arguments proposed, that the conventions to settle the post-Nazi order in Europe would likely at best only support a return to the population situation of the 1920s or to a settlement based on that situation, which would mean that the Kola peninsula and some parts of Karelia would be returned to Russia to allow for those expelled to return home and that other areas might be subject to a plebiscite based on the surviving pre-war population (including those still resident and the expellees) voting on what they wanted for the future of the territory (though I would expect Russia and Finland to heavily dispute this with Russia calling for descendants of expellees to be be considered part of the electorate for the plebiscite while Finland would want to include any settlers in the vote or at least any persons born in the territories whether to residents or settlers - this dispute over who should be eligible to vote could be one of those sticking points that keeps the dispute simmering rather like the Western Sahara dispute).