Germany went through many changes to its borders during the 19th and the 20th century, with consequential effects on German citizenship.
Following WWI and WWII, the borders of the country have shifted to the point that many people – who used to live in what was then Germany – found themselves citizens of a different country.
These countries include (but are not limited to) Poland, Belarus, Ukraine, the Czech Republic, Slovakia, Lithuania, Latvia and Estonia and the now Russian area of Kaliningrad.
As a general rule, the German Basic Law of 1949 set out the terms through which former German citizens had the chance to reclaim their nationality, which might have however already been lost and reclaimed since 1918.
Following WWI, for example, the Treaty of Versailles made provision for all the German and Polish citizens living in the affected areas to decide which nationality they wanted to maintain following the change in the borders.
Over the years, so-called ‘German Poles’ were able to reclaim their German nationality, if they wished to do so. At the same time, former Germans who re-established their residence in Germany after 8th May 1945 were not considered to have lost their nationality at all.
Considering the vast number of people who then reclaimed their German nationality – and out of fear of a potential detachment of a German-citizen-filled area from the Polish territory – Germany and Poland struck a deal in 1990, through which they declared the borders (as they were) to be fixed.
The question as to whether these people (and their descendants) had or have a claim for German citizenship is a very complicated one and will need to be established on an individual basis after research into various laws and treaties.