I don't remember where I learned about it (I've read a bunch of books about fascism over the last few years, so probably one of those!) and it's not really a secret (as I mention in the piece the wikipedia article is very good.)
just one of those things where he's very well known for the one thing and unless you're a historian of the period you probably don't know a lot more, I guess.
I for one had no idea what his background looked like. That being said, it makes his poem even more meaningful imo. Shows that fascists don’t care if you agree with them on some things, you have to agree on all fronts or you are the enemy.
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https://en.m.wikipedia.org/wiki/Martin_Niem%C3%B6ller