I will describe my own national case. Both the family of my mother and the family of my father, originally came from the region of Pomerania, so from the Kingdom of Prussia. Father's family from area of modern Koszalin (in Western Pomerania), mother's family from area of modern Bydgoszcz / Toruń.
My paternal great-grandfather emigrated in the 19th century to the USA. My grandfather was born in American in 1895, and my father was also born here in 1920, as de facto 3rd generation American person. But in 1922 my grandfather to return to the country of "origins" (my father was 2 years old then) and after returning he settled near Stettin (so we know, what country of "origins" I am talking about). But in 1928 grandfather bought an estate near Wegrów in the vicinity of Warsaw and moved there together with his family (I don't know what were the reasons of that international migration fro the USA to Weimar Germany and several years later to Poland). The family of my father (including his "American" siblings), lives near Warsaw until today, and they consider themselves as "pure Poles".
Family of my mother also consider themselves "purely Polish".
When it comes to me, on the other hand, even though I was born in Poland, I have always considered myself American and during the first occasion (and in Communist Poland those occasions were not so frequent), I returned to the country of my ancestors - to the USA.
But let me complicate the case even more! My present wife comes from Rybnik... She is an American citizen today, obviously, but she considers herself Polish. And this is despite the fact that parents of my wife lived for many years in Munich, because they had emigrated to Germany as "Germans". What is even funnier - grandfathers of my wife considered themselves as ethnic Polish Silesians, but her great-grandfathers considered themselves as "ethnically pure Germans"!
Paranoia!
But let me continue...
Adult daughter of my wife has lived for years in Great Britain (she has British citizenship), but she is married to an American citizen, so I have no idea, who my 6 years old grand daughter is going to be...
Son of my wife lives in Poland in Rybnik, and if you call him "Silesian", he is going to kick you out of his house! According to him, he is "totally Polish".
But this is not all!
My adult son - from my first marriage... also with a Polish woman, because we had emigrated together to the USA (...) lives in the USA, but he was born in... Hamburg!
(...)
My son considers himself American, just like me!
I have a sentiment for Poland, as a country of my childhood and youth, but I do not consider myself Polish.
However, my older sister who lives in Poland is totally Polish and doesn't want to visit the USA... but her brother lives here and considers himself Polish-American (he has double American and Polish citizenship, I have only American).
My paternal grandmother, never learned to speak Polish properly and I remember that when I was a small child, I was "afraid" of her, because it was hard to understand her (I talked to her later in English). She was native American, even though her roots were from England, specifically from Kent.
In my home, as children we talked to each other in Polish, even though parents demanded from us to "officially" talk in English from time to time.
Everyone will admit, that it would be hard to find a more complicated situation!
(...)
As you can see from my own example, issues concerning national identification, as well as issues of "ancestry", are more complicated in reality, than one would think.