Furthermore, for a "white nationalist" movement they spent a whole lot of time killing and oppressing other "whites".
Communism is problematic to them because it tries to eliminate social hierarchies deemphasizing national identity. Liberalism (in the classical sense) is problematic to them because of its (perceived) amoral decadence and its ability to empower "undesirable" "outsider" individuals to garner power inside a nation through wealth.
If that was the case, then why did their policies include the murder of millions of German-speaking Jews, Gypsies, and people with disabilities. What was it about those people that made them not fit the category of "German"?
The answer from clearly documented history is that the Nazis systematized the categorization of humans on their deeply flawed model of the Aryan "race", and singled out those who didn't fit for subjugation or extermination.
Conversely they also planned to further "purify" their own people by planning to subject Scandinavian populations to forced interbreeding programs upon conquering them, and kidnapping "Aryan" children from neighboring countries.
See Generalplan Ost.
Because Jews and Gypsies weren't considered ethnically German. Re: disabilities, it's a mix of rabid utilitarianism (they were called "useless eaters") and a belief that disabilities stemmed from corrupted bloodlines (hence the term mongoloid -- it was a popular theory that retardation was the result of Europeans/Asian intermarriage).
What defined German ethnicity?
Clearly it wasn't the ability to speak the language, or being descended from many generations of ancestors living in Germany, otherwise the Jews and Roma both would have been considered German.
Clearly there was something else attributed to them that didn't fit the definition of "German". It was that they weren't considered racially German. Race is literally what the Nuremberg Laws were about:
https://en.wikipedia.org/wiki/Nuremberg_Laws
https://en.wikipedia.org/wiki/Racial_policy_of_Nazi_Germany
Given the preponderance of clear historical evidence for the Third Reich's racially based rationale for its actions, one wonders why would anyone try to water it down to the more innocuous seeming rationale of "German ethnicity".
I'm not making that up - it was right there in their laws that I linked to.
Otherwise, why shouldn't people who had been living in Germany for generations, speaking languages closely related to German (Yiddish, German Romani), be considered German?
How can you interpret those laws as being about anything but racial categorization - unless you have an agenda to sidestep and downplay the fact the Nazis repressed and murdered people whom they saw as not being part of their own presumed "race".
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?
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).