I'm hearing this argument over and over again, yet for some reason no one can explain whether giving up land to an invader is always unacceptable, or are there some exceptions? Maybe it's how long ago the land was taken?
I suspect it's the latter, and this time period is greater than 10 years (because Crimea is still "rightfully Ukrainian", right?), but less than a few hundred years (because all the other invaded territories now "rightfully belong to the invader".
You do understand that almost every piece of land on this planet, that belongs to country A, was conquered from another country B (or from a tribe B if we go long enough into the history, or from family B if go even further, or from some other entity if we go even further)?
Should Poland invade Ukraine to retake Lviv?
Should Finland invade Russia to retake Karelia?
Should Mexico invade USA to retake California?
Should every country in Europe invade every other country in Europe because they all hold some territories that belonged to a different country some time back?