It's also why it's not easily dispatched by enlightenment thinking... it survived those battles back in the 20th century and ultimately had to be put down with exercises of power (often military, sometimes social, rarely diplomatic).

Fascists gained power in Italy, Germany, and Spain as a direct result of anti-communist sentiment (and in the case of Germany, a failed communist revolution). Mussolini's early writings were directly proposing nationalism as the answer to communism's failures. Everywhere fascism had success it was because it was pitted against the threat of communism. I don't know how the rise of Fascism was anything but reactionary.

But we're talking about the root causes of WWII. And one of the Nazi party's specific goals when invading Poland was to exterminate the Jews and Roma (Gypsy) populations of Europe.

Are you sure about that? Wasn't "the final solution" proposed significantly later?