Hmm? I think you've got that backwards. Ukraine is the one trying to reclaim lost territory that's currently under Russia's control, is it not? What year exactly should we revert world borders back to and why?

I wonder if you can see the problem with the naive solution of trying to "lock in" whatever the present borders are. If a country seizes territory, even without any justification, that territory is now part of the present borders, and therefore would be "locked in" by that standard, suggesting that anyone who tried to take it back is the aggressor (until they succeed in reclaiming it).

I think that what you're asking is a very complicated and valid question, even if you didn't mean it in earnest. The question of what makes a country legitimate is quite complicated. I would argue that the "north star" of legitimacy is what outcome is best for the people. In the case of Taiwan, I think the best outcome is to maintain the status quo of de facto independence without rocking the boat with things like formal independence. It's not worth starting world war 3 over a formality.

But when you have a "country" like the Confederacy or Tibet, which keeps people in bondage under horrible conditions, then obviously the best outcome is for them to be defeated and taken over by someone else. Slavery and serfdom are automatically delegitimizing.

There's also another reason why reunifying Tibet was justified, which is explained very succinctly by the 1944 US War Department film, "Why We Fight: The Battle For China:" (around 8:20)

But how could Japan, only 1/20th the size of China, and with only 1/6th it’s population, think of conquering China, much less the world?

Modern China, in spite of its age old history, was like the broken pieces of jigsaw puzzle, each piece controlled by a different ruler, each with his own private army. In modern terms, China was a country, but not yet a nation.

The part of China's history where it was broken up into these warlord states was part of what they call, "The Century of Humiliation," when Chinese people were subject to imperialism and aggression from many different countries, worst of all being Imperial Japan. Because the country was so fractured, it was difficult to mount an organized, collective defense. This was understood by basically everyone, by the US, by the communists, and by the nationalists. That's why the communists and nationalists were willing to form a unified front against the warlord states despite their major ideological differences, because it was obvious to everyone at that time that a unified China - a "One China Policy" - was important and necessary. Even today, both the PRC and ROC formally agree on the idea of a One China Policy, and the US has (in the past at least) as well.

But again, today, I personally believe in maintaining the status quo, where Taiwan is de facto independent. There's significant precedent that this can maintain peace and keep everyone relatively satisfied. The same precedent did not exist in Tibet or in any of the other warlord states. Furthermore, Taiwan has significantly better human rights and conditions in general than Tibet where you'd die a serf at age 30. The whole "Free Tibet" thing is pure propaganda, only followed by people who are completely ignorant of the actual facts of what life was like there before, and of the history in general.