What is the origin of white supremacist obsession with Nordic peoples?

by PhoenicianPirate

One thing that I have observed for decades is that American white supremacists and Nazis were completely obsessed with the idea that Nordic and northern European peoples were the master race. I know that this belief started somewhere in the 19th century. But what exactly is the origin of it, why did it get rooted so deeply in their mindset, and why do they believe that blond nords are the master race? Even some people who aren't Nordic.

voyeur324

/u/killfile has previously answered Why was being Aryan such a big deal for the Germans? as a follow-up to a question about the swastika.

/u/jan_van_bergen has previously written about the Nazi attachment to the word 'Aryan'