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In the time after WWI, Germany was a relatively liberal country in some respects. The salon society was popular in Berlin and Gays and Lesbians were able to live their lives in relative freedom- not the freedom we have now in the USA, but not the level of persecution they would experience elsewhere. There were magazines that catered specifically to this population as well as private clubs for them to congregate.
Of course these organizations were then easily used by the Nazis to find all of the gays and lesbians in germany...
One thing that makes me really angry about most popular accounts of that regime is the convenient forgetting that Gays and Lesbians were part of the holocaust and were very likely to be straight up slaughtered rather than sent to camps. And while there were fewer gays killed than jews (smaller part of the population) there are other groups as well that are often forgotten... in fact I can only think of the Roma. Shouldn't we all know all the groups the Nazis systematically murdered?
GUNS
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The important operational part of this is that the data was collected at a time when it seemed innocent and then the regime changed and the data was used to kill people. The nazis are, of course, a very extreme example, but regimes change all the time. For instance, a persecuted group in the USA right now are gun owners. (Yes, I know that guns are used wrongly, over 30 years nearly 600 people have been killed in mass shootings-- of course that doesn't compare to the 10,000 people killed each year by drunk drivers.)
Increasingly, one of the tactics used to go after gun owners is "registration". After all, if you're law abiding, why should you fear registering your guns? There is a massive program right now that causes "FBI Background Checks" for every gun purchase--and by this nature this is registration of firearms and firearms owners.
GERMS
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So maybe you hate guns and gun owners so you see nothing wrong with keeping track of them. The thing is, when you live long enough (Say into your 40s) you start to see how attitudes change. Some for the better-- gays are less persecuted now, and I'm grateful for that (being not exactly heterosexual myself). But you also see other groups being persecuted. This starts out seemingly innocently enough. Lets take Muslims. They have been linked to real crimes and that has been used to smear a whole group. (There are other groups getting the same treatment right now- gamers, men, christians, etc.)
The germ, which becomes an infection and spreads, is thinking that because a person of type X committed a crime (A jew in germany charged too much? a boy at college committed rape? a muslim killed people in a terrorist act?) ... that the whole group can be painted with the brush of prejudice.
That prejudice becomes systematic and eventually it can become oppression. Often it becomes the situation where those who think they are oppressed justify oppressing others because they claim their victims are oppressing them. (You see this now with the more extreme elements of feminism. No, not all men are rapists! Really. Some of them are gay!)
This is exactly what happened in Nazi Germany where the county had been victimized by the treaty that ended WWII and humiliated and felt widespread victimhood.
So, beware of victimhood being used to justify prejudice!
That's one lesson people don't seem to learn from this era-- its the mechanism by which a peaceful, tolerant society (as germany had) becomes fascist.
After all, germans are not genetically predisposed to fascism, any more than americans are.