Originally Posted by
televisor
I believe most european citizenships are hereditary. That likely means there are many European citizens in the US unaware of their citizenship (unless their ancestors explicitly renounced citizenship).
Emphasis on the fact that they are (legally seen) citizens, not that they are eligible for citizenship. In most cases there won't be any documentation to prove this, so these people are de-facto non-citizens, but if it were tracked accurately some people could have a whole bunch of european passports.
(E.g. polish citizenship is hereditary: it's passed on to all children of a polish parent, regardless of this citizenship being documented.)
Out of interest, did/does Luxembourg not pass on citizenship by birth and/or do they automatically remove citizenship if it's not claimed/documented? (I know some other countries automatically do if you don't register yourself with a consulate within 20-25 years of birth, if living abroad.)
Many European countries prohibited dual-citizenship, and so citizenship by descent of people born in the Americas of people who were born in or migrated to the US in the 1800s and a good chunk of the 1900s don't have a valid claim to European citizenship by descent. Add in that a bunch of European countries had changing borders and changed citizenship laws over time, along with the reality that an oath of US citizenship or engagement in US military/government employment was at some times considered an act of relinquishment of non-US citizenship. It, with other things, all makes for a rather complicated picture whennspeaking of so many countries. But there are indeed many millions of US citizens who have a valid claim to European citizenship but don't all know it and pursue it.
For example while Polish citizenship is indeed available by descent, the picture is rather more complicated than that everyone who is descended of some Polish citizen at some point in time is necessarily a Polish citizen.
Luxembourg has citizenship by blood and it doesn't appear that a registration of citizenship had to necessarily take place even before death.