Attached below is an in-depth introduction to the history of Germany, from its barbarian time till now. It is worth your while to read this a little long report at: 

德国版库页岛：德国国歌提及的4个地名，居然全在德国境外？_风闻 (guancha.cn)

or you can read the English version at:

The German version of Sakhalin: The four place names mentioned in the German national anthem are all outside Germany? (guancha.cn)