Russia annexet Crimea because "it used to be Russian". China is threatening Taiwan because they see it as part of China. The Falkland islands have been disputed for some time, and even invaded by Argentina and then taken back by the UK. Meanwhile Sweden isn't trying to reclaim parts of Estonia nor does Sweden try to retake Finland and Norway. And Denmark isn't trying to reclaim the south of Sweden despite it historically being German. Is there a historical reason to why some countries seem to obsessed by historical borders whereas others are not?
In the case of Rusia, really the “ use to be Russian “ is more a excuse to invade it, as in Crimea is Sebastopol wich is a warm port in the Black Sea so it’s very important. China is different as the aren’t reclaiming a land from another country that use to be Chinese, they are reclaiming that That the People’s Republic of China ( the China that we all know ) is the real China, and that the Republic of China ( aka Taiwan ) is therefore part of it, in fact Taiwan was “ formed “ when the government of China fled to Taiwan to escape the communist. And Argentina claim is principally because of the Falkland war, Witch was started to bust moral in the country as they were in a militar dictatorship.