OP is asking an interesting and important question. While I don’t claim it as definitive, there was an insightful article written in 2016 that attempted to address the question head on: How Half Of America Lost Its F**king Mind

Ah. This is all a joke to you. Sorry for having taken you seriously.

May I ask, if someone were to make a gesture indistinguishable from a Nazi salute, what would be necessary to convince you it was, in fact, a Nazi salute?