One of the more interesting articles published last year:
Q: You, a black man, sat in on a white-separatist retreat. How did that go over?
A: They were curious and shocked they had found a black man on their premises. A lot of the members of the church took pains to explain to me the difference between white supremacy and white separatism. They said, "We don't think we're better than you; we just want to be separate from you."
Racism is wanting to make definitions about which races are superior, and which are inferior.
Nationalism is wanting to be with people like you: culture, heritage, values, language and customs.
Underlying all of these is (a) similar raw ability and (b) similar parallel evolution, meaning that your people have similar tastes, tendencies and inclinations. Instinct is shaped by evolution and is the cornerstone of culture and values.
I use "race" for ethnicity in the above because it's how we speak of it. While I think both race and ethnicity/ethny are valid concepts, I think ethnicity is the more precise term.
Racism is caused by diversity, or ethnic competition where ethnic segregation is disallowed. The response is reactionary and violent.
Nationalism is the opposite of racism -- it is tolerance, in parallel, for all ethnic groups, including the right to preserve themselves by segregating. After all, if we force other groups upon them, they'll be bred into a hybrid, and the original group will be destroyed. That's racism and genocide.
If we do not suppress the urge to segregate, whether by Black Panthers or White Wolves, I think we'll all be OK.