Nazis
In order to reduce amount of potential strawmans, I feel like I need to at least functionally define the thing I wish total extintion upon. Here.
Nazi is anyone who believes that reinforcing sense of national identity is desirable.
We do not want to confuse "national" with whatever anything else though; it's important to draw the line between three similar concepts, namely: citizenship, ethnicity and nationality. The first two are objective, the last one is purely ideological.
The word citizenship describes relationship of the person with a state. While the state does not exist physically or objectively, written documents do. Whatever is written in you passport (or passports) is your active citizenship(s). Nobody will argue with that.
Definitions of ethnicity are much more shady and speculative, yet no matter which parameters you would decide to group people by, they can still be measured somehow.
Color of your skin can be documented, language you learn first can remain the most pleasant to your ear and tongue through entire life, and you can have some sort of emotional connection to some place on earth due to various reasons.
To sum it all up:
You can technically talk about ethnicities and citizenships in scientific context.
This is where the word "nationality" stands out. Since it was initially invented to make soldiers more closely identify with their own army and not with an enemy, it is by its nature always forceful, exclusive, and most importantly, political.
Nobody will have a problem with you showing multiple passports proving your multiple citizenships. But you can't ever convincingly pass as a holder of few different nationalities at the same time.
Absence of nationality is also impossible to present. The idea is so deeply engraved in people's minds that the first thing they do seeing you is labeling you with some nationality. They feel deeply uncomfortable not knowing it, so they will assume, even if you don't tell.
I've had those conversations countless times, just enought to tell that everyone has that kind of prejustice about you in mind before you even talk to them. And if the assumptions were wrong (you show not the passport they expect), they justify their initial thoughts by assuming you belonging to some ethnicity genetically.
Nationality is always in the eye of beholder.
It's not a coincidence that governments start to talk about nationality much more when they are at war or short on resources. The goal of such blatant manipulation is to make you honestly and willingly give up your own inetests and stand for the govenment, for "the home", for "the people". The only reason fo such idea to exist is to incite you to act in a way you would not act otherwise.
A manipulative and authoritarian conception to the core, it has no right to exist in a healthy, sane society. Nazis think they are entitled to tell other people what to do.
They are not.